Add workflow action to setup Python 3.8

[jacobperron]

Attempt to force CI to use Python 3.

The workflow is failing, for example:

...
 -- Found PythonInterp: /usr/bin/python (found version "2.7.18") 
  CMake Error at /opt/ros/rolling/share/rosidl_adapter/cmake/rosidl_adapt_interfaces.cmake:60 (message):
    execute_process(/usr/bin/python -m rosidl_adapter --package-name
    domain_bridge --arguments-file
    /home/runner/work/domain_bridge/domain_bridge/ros_ws/build/domain_bridge/rosidl_adapter__arguments__domain_bridge.json
    --output-dir
    /home/runner/work/domain_bridge/domain_bridge/ros_ws/build/domain_bridge/rosidl_adapter/domain_bridge
    --output-file
    /home/runner/work/domain_bridge/domain_bridge/ros_ws/build/domain_bridge/rosidl_adapter/domain_bridge.idls)
    returned error code 1:

    Traceback (most recent call last):

      File "/usr/lib/python2.7/runpy.py", line 163, in _run_module_as_main
        mod_name, _Error)
      File "/usr/lib/python2.7/runpy.py", line 111, in _get_module_details
        __import__(mod_name)  # Do not catch exceptions initializing package
      File "/opt/ros/rolling/lib/python3.8/site-packages/rosidl_adapter/__init__.py", line 32
        assert False, f"Unsupported interface type '{interface_file.suffix}'"
                                                                            ^

    SyntaxError: invalid syntax

[jacobperron]

hmm, I initially thought that it may be an issue with the GitHub workflow, but I think this is a regression caused by https://github.com/ros2/rosidl/pull/612

[jacobperron]

Ah, okay. We are installing rosidl_parser 2.5.0, but we need rosidl_parser 3.0.0. I hope the issue will resolve itself with the next Rolling sync to testing. Currently, it is blocked by some other package build failures: https://build.ros2.org/job/Rrel_sync-packages-to-testing_focal_amd64/