Using ROS Melodic with Python 3

edit

I'm trying to install ROS M following the installation instructions,. I installed the desktop configuration, but when tried step 1.6, rosdep installation, it doesn't work for python3-rosdep. Is this the right way of doing it?

I'm using Ubuntu 18.04

$ sudo apt install python3-rosdep
[sudo] password for user: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 python3-rosdep : Depends: python3-catkin-pkg but it is not going to be installed
                  Depends: python3-rosdistro but it is not going to be installed
                  Depends: python3-rospkg but it is not going to be installed
                  Depends: python3-rosdep-modules (>= 0.18.0) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

EDIT:

If I try to install python3-rospkg it will attempt to remove a lot of ros-* packages I just installed