Cast pathlib.Path to str inside normalize_to_list_of_substitutions.

[Interpause]

This resolves #805. More details can be found there.

After backporting the commit to humble, I have tested it to at least work on my launch files, allowing me to remove the str() cast.

Since this is my first time, I am not exactly sure how to run tests, but I think this means its good?

============================================================================================= test session starts =============================================================================================
platform linux -- Python 3.10.12, pytest-6.2.5, py-1.10.0, pluggy-0.13.0
rootdir: /workspaces/project-name/thirdparty/launch/launch_pytest, configfile: pytest.ini
plugins: launch-pytest-3.6.1, launch-testing-3.6.1, ament-copyright-0.12.11, ament-xmllint-0.12.11, launch-testing-ros-0.19.7, ament-lint-0.12.11, ament-flake8-0.12.11, ament-mypy-0.12.11, ament-pep257-0.12.11, colcon-core-0.18.1, cov-3.0.0
collected 18 items                                                                                                                                                                                            

test/test_copyright.py .                                                                                                                                                                                [  5%]
test/test_flake8.py .                                                                                                                                                                                   [ 11%]
test/test_pep257.py .                                                                                                                                                                                   [ 16%]
test/launch_pytest/test_plugin.py .............                                                                                                                                                         [ 88%]
test/launch_pytest/tools/test_process.py ..                                                                                                                                                             [100%]

============================================================================================= 18 passed in 17.48s =============================================================================================

EDIT:
Also would it be possible to backport this to humble? Thanks

[Interpause]

Just saw #790. I think that one seems better as its more thorough & explicit, though that means more review work needed. Mine is closer to a quick workaround.

EDIT: I missed the case of putting Path inside a list of substitutions too it seems