Remove pytest-runner

For the pytest-runner removal TODO.

This edits the following texts:

$ grep pytest-runner setup.py requirements.txt 
setup.py:    # Pin pytest-runner to 5.2.0, since 5.3.0 uses `find_namespaces` directive, not supported in
setup.py:    'pytest-runner == 5.2.0',
requirements.txt:pytest-runner

The version comparison doesn't pose as a problem because pytest is 7.x, so I thought it would be cleaner then adding a patch or other more complex text-editing statements.