I apologize for posting this question as python3 is not entirely supported for ROS 1. I am using a python3 library (it uses int.frombytes and int.tobytes) in my project. I have successfully installed python3 on Raspberry Pi as well. I need to run ROS (Indigo) node on this board. I found brief instructions for python 3 in ROS Indigo here. Unfortunately, it didn't work quite well, or I missed some step.

The answer referred above says to create a virtual environment by using virtualenv -p /usr/bin/python3 and then under it run pip install rospkg. It didn't work. Please see terminal below-

(whill) pi@raspberrypi:~ $ ipython
Python 3.7.3 (default, Apr  3 2019, 05:39:12) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.11.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import rospy                                                            
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
<ipython-input-1-fb60b5b51009> in <module>
----> 1 import rospy

ModuleNotFoundError: No module named 'rospy'

Next, I followed the official Wiki page for installing ROS Indigo on the Raspberry Pi here. Please note that all these instructions were executed inside the python3 virtual environment.

From ROS Indigo, I need only publishers and subscribers of standard ROS messages. Therefore I decided to install ROS-Comm by using the following commands-

(whill) pi@raspberrypi:~/ros_catkin_ws $ /home/pi/whill/bin/rosinstall_generator ros_comm --rosdistro indigo --deps --wet-only --exclude roslisp --tar > indigo-ros_comm-wet.rosinstall
(whill) pi@raspberrypi:~/ros_catkin_ws $ /home/pi/whill/bin/wstool init src indigo-ros_comm-wet.rosinstall

I also installed libconsole-bridge-dev as stated in the Wiki page. The remaining dependencies were resolved by running rosdep-

(whill) pi@raspberrypi:~/ros_catkin_ws $ /home/pi/whill/bin/rosdep install --from-paths src --ignore-src --rosdistro indigo -y -r --os=debian:buster

The above took a while to finish. Finally, the catkin workspace was build as shown below-

(whill) pi@raspberrypi:~/ros_catkin_ws $ sudo ./src/catkin/bin/catkin_make_isolated --install -DCMAKE_BUILD_TYPE=Release --install-space /opt/ros/indigo -j2
Base path: /home/pi/ros_catkin_ws
Source space: /home/pi/ros_catkin_ws/src
Build space: /home/pi/ros_catkin_ws/build_isolated
Devel space: /home/pi/ros_catkin_ws/devel_isolated
Install space: /opt/ros/indigo
Additional CMake Arguments: -DCMAKE_BUILD_TYPE=Release
Additional make Arguments: -j2
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~  traversing 48 packages in topological order:
~~  - catkin
~~  - genmsg
~~  - gencpp
~~  - genlisp
~~  - genpy
~~  - cmake_modules
~~  - cpp_common
~~  - message_generation
~~  - message_runtime
~~  - mk
~~  - ros
~~  - ros_comm
~~  - rosbash
~~  - rosboost_cfg
~~  - rosbuild
~~  - rosclean
~~  - roscpp_traits
~~  - roscreate
~~  - rosgraph
~~  - roslang
~~  - rosmake
~~  - rosmaster
~~  - rosmsg
~~  - rospack
~~  - roslib
~~  - rosparam
~~  - rospy
~~  - rosservice
~~  - rostime
~~  - roscpp_serialization
~~  - roslaunch
~~  - rosunit
~~  - rosconsole
~~  - roslz4
~~  - rosbag_storage
~~  - rostest
~~  - std_msgs
~~  - rosgraph_msgs
~~  - std_srvs
~~  - xmlrpcpp
~~  - roscpp
~~  - rosout
~~  - message_filters
~~  - rosnode
~~  - rostopic
~~  - roswtf
~~  - topic_tools
~~  - rosbag
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The packages or cmake arguments have changed, forcing cmake invocation

==> Processing catkin package: 'catkin'
==> Creating build directory: 'build_isolated/catkin'
==> cmake /home/pi/ros_catkin_ws/src/catkin -DCATKIN_DEVEL_PREFIX=/home/pi/ros_catkin_ws/devel_isolated/catkin -DCMAKE_INSTALL_PREFIX=/opt/ros/indigo -DCMAKE_BUILD_TYPE=Release -G Unix Makefiles in '/home/pi/ros_catkin_ws/build_isolated/catkin'
-- The C compiler identification is GNU 8.3.0
-- The CXX compiler identification is GNU 8.3.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Using CATKIN_DEVEL_PREFIX: /home/pi/ros_catkin_ws/devel_isolated/catkin
-- Using CMAKE_PREFIX_PATH: 
-- Found PythonInterp: /usr/bin/python (found version "2.7.16") 
-- Using PYTHON_EXECUTABLE: /usr/bin/python
-- Using Debian Python package layout
-- Using empy: /usr/bin/empy
-- Using CATKIN_ENABLE_TESTING: ON
-- Call enable_testing()
-- Using CATKIN_TEST_RESULTS_DIR: /home/pi/ros_catkin_ws/build_isolated/catkin/test_results
-- Found gtest: gtests will be built
-- Using Python nosetests: /usr/bin/nosetests-2.7
-- catkin 0.6.19
-- Configuring done
-- Generating done
-- Build files have been written to: /home/pi/ros_catkin_ws/build_isolated/catkin
==> make -j2 in '/home/pi/ros_catkin_ws/build_isolated/catkin'
==> make install in '/home/pi/ros_catkin_ws/build_isolated/catkin'
COMPLETE OUTPUT IS NOT SHOWN TO KEEP IT SHORT

The build process failed due to a missing header file from the boost. Please see below-

+ cd /home/pi/ros_catkin_ws/src/ros_comm/rosmsg
+ mkdir -p /opt/ros/indigo/lib/python2.7/dist-packages
+ /usr/bin/env PYTHONPATH=/opt/ros/indigo/lib/python2.7/dist-packages:/home/pi/ros_catkin_ws/build_isolated/rosmsg/lib/python2.7/dist-packages:/opt/ros/indigo/lib/python2.7/dist-packages CATKIN_BINARY_DIR=/home/pi/ros_catkin_ws/build_isolated/rosmsg /usr/bin/python /home/pi/ros_catkin_ws/src/ros_comm/rosmsg/setup.py build --build-base /home/pi/ros_catkin_ws/build_isolated/rosmsg install --install-layout=deb --prefix=/opt/ros/indigo --install-scripts=/opt/ros/indigo/bin
running build
running build_py
running build_scripts
running install
running install_lib
running install_scripts
changing mode of /opt/ros/indigo/bin/rosmsg to 755
changing mode of /opt/ros/indigo/bin/rosmsg-proto to 755
changing mode of /opt/ros/indigo/bin/rossrv to 755
running install_egg_info
Removing /opt/ros/indigo/lib/python2.7/dist-packages/rosmsg-1.11.21.egg-info
Writing /opt/ros/indigo/lib/python2.7/dist-packages/rosmsg-1.11.21.egg-info
<== Finished processing package [23 of 48]: 'rosmsg'

==> Processing catkin package: 'rospack'
==> Building with env: '/opt/ros/indigo/env.sh'
Makefile exists, skipping explicit cmake invocation...
==> make cmake_check_build_system in '/home/pi/ros_catkin_ws/build_isolated/rospack'
==> make -j1 in '/home/pi/ros_catkin_ws/build_isolated/rospack'
[ 11%] Building CXX object CMakeFiles/rospack.dir/src/rospack.cpp.o
In file included from /home/pi/ros_catkin_ws/src/rospack/src/rospack.cpp:28:
/home/pi/ros_catkin_ws/src/rospack/include/rospack/rospack.h:108:10: fatal error: boost/tr1/unordered_set.hpp: No such file or directory
 #include <boost/tr1/unordered_set.hpp>
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [CMakeFiles/rospack.dir/build.make:63: CMakeFiles/rospack.dir/src/rospack.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:371: CMakeFiles/rospack.dir/all] Error 2
make: *** [Makefile:141: all] Error 2
<== Failed to process package 'rospack': 
  Command '['/opt/ros/indigo/env.sh', 'make', '-j1']' returned non-zero exit status 2

Reproduce this error by running:
==> cd /home/pi/ros_catkin_ws/build_isolated/rospack && /opt/ros/indigo/env.sh make -j1

Command failed, exiting.

It seems that I am using boost 1.67 as shown below-

(whill) pi@raspberrypi:~/ros_catkin_ws $ apt-cache policy libboost-dev
libboost-dev:
  Installed: 1.67.0.1+b1
  Candidate: 1.67.0.1+b1
  Version table:
 *** 1.67.0.1+b1 500
        500 http://raspbian.raspberrypi.org/raspbian buster/main armhf Packages
        100 /var/lib/dpkg/status

Below is the information about Raspberry Pi OS-

(whill) pi@raspberrypi:~/ros_catkin_ws $ cat /etc/os-release
PRETTY_NAME="Raspbian GNU/Linux 10 (buster)"
NAME="Raspbian GNU/Linux"
VERSION_ID="10"
VERSION="10 (buster)"
VERSION_CODENAME=buster
ID=raspbian
ID_LIKE=debian
HOME_URL="http://www.raspbian.org/"
SUPPORT_URL="http://www.raspbian.org/RaspbianForums"
BUG_REPORT_URL="http://www.raspbian.org/RaspbianBugs"

pi@raspberrypi:~ $ uname -a
Linux raspberrypi 4.19.75-v7+ #1270 SMP Tue Sep 24 18:45:11 BST 2019 armv7l GNU/Linux
pi@raspberrypi:~ $ gcc --version
gcc (Raspbian 8.3.0-6+rpi1) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

pi@raspberrypi:~ $ g++ --version
g++ (Raspbian 8.3.0-6+rpi1) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

pi@raspberrypi:~ $ cmake --version
cmake version 3.13.4 
CMake suite maintained and supported by Kitware (kitware.com/cmake).

pi@raspberrypi:~ $ python --version
Python 2.7.16

pi@raspberrypi:~ $ python3 --version
Python 3.7.3

The rosinstall file can be seen online here. My aim is to use Python 3 in ROS Indigo (only publisher/subscriber for std_msgs) node on the Raspberry Pi board.