How do you use Python3 and its libraries with ROS Melodic (researched)?

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Hi, I'm a total ROS newbie and I'm looking for help with integrating SPADE multi-agent Python library (3.6 and up) with ROS Melodic (Ubuntu 18.04). I'm really desperate right now because I was looking for many different anwsers how to do such a thing. This is a precise description of what I did and with many many different attempts before:

Creating the main package:

1. I create folders catkin_ws/src and run the command catkin_make in catkin_ws to create a workspace.
2. I create the first package named spade_test in which the main .py files will be located, I use catkin_create_pkg spade_test std_msgs rospy in the src folder.
3. Every time I change something significant, I run catkin_make and . ~/catkin_ws/devel/setup.bash commands.
4. I add <build_depend>message_generation</build_depend> and <exec_depend>message_runtime</exec_depend> to package.xml file and message_generation in find_package section and CATKIN_DEPENDS message_runtime in catkin_package section of CMakeLists.txt file. I also uncomment the generate_messages section (don't think anything in this step is essential).
5. I add 2 files talker.py and listener.py to the spade_test folder, the same code as in this tutorial.
6. I'm using the chmod + x filename.py command to make these work as executable files.

Creating package with a library inside:

1. I create the spade package by using the catkin_create_pkg spade std_msgs rospy command in the src folder.

2. In the package.xml file in the <version> tag, I change it to 3.1.2, while in the CMakeLists.txt file I uncomment the catkin_python_setup() line.

3. I git clone everything from the SPADE repository to the spade folder (setup.py file included).

4. In spade_test package's package.xml file I add 3 lines: <build_depend>spade</build_depend>, <build_export_depend>spade</build_export_depend> and <exec_depend>spade</exec_depend>. In its CMakeLists.txt I add spade to find_package section.

5. After using catkin_make and . ~/catkin_ws/devel/setup.bash commands, the talker.py and listener.py files work fine after using rosrun spade_test talker.py etc.

6. I added from spade.agent import Agent to test it if the library works properly and sadly it doesn't, the error occured:

Error

Traceback (most recent call last):
  File "/home/my_name/catkin_ws/src/spade_test/talker.py", line 6, in <module>
    from spade.agent import Agent
  File "/home/my_name/catkin_ws/devel/lib/python2.7/dist-packages/spade/__init__.py", line 35, in <module>
    exec(__fh.read())
  File "<string>", line 3, in <module>
  File "/home/my_name/catkin_ws/src/spade/spade/agent.py", line 86
    async def _async_start(self, auto_register=True):
            ^
SyntaxError: invalid syntax

It's definitely related to the wrong version of Python my ROS is using (2.7) and I was looking everywhere to change it (even some answers from this place) but sadly nothing has helped. I also created another catkin workspace with catkin build --cmake-args -DPYTHON_VERSION=3.6 command instead of catkin_make with the exact same result (except with /home/my_name/catkin_ws/devel/lib/python3/dist-packages/spade/__init__.py line in the error instead).

I installed my ROS Melodic with this tutorial ... (more)