Drop support for Python 3.6 in GitHub workflows

[bemoody]

Python 3.6 is officially end-of-life and is no longer supported in GitHub workflows on ubuntu-latest.

I guess that this is the case since ubuntu-latest was updated to Ubuntu 22.04: https://github.com/actions/runner-images/issues/6399

Alternatively, if folks want to continue supporting Python 3.6 in this package, the test suite could be changed to use an older Ubuntu release.

[bemoody]

Also flake8 is broken on "macos-latest", python 3.7:

2023-01-05T20:38:51.9917260Z ##[group]Run flake8
2023-01-05T20:38:51.9917820Z flake8
2023-01-05T20:38:52.0400760Z shell: /bin/bash -e {0}
2023-01-05T20:38:52.0401050Z env:
2023-01-05T20:38:52.0401580Z   pythonLocation: /Users/runner/hostedtoolcache/Python/3.7.15/x64
2023-01-05T20:38:52.0401920Z ##[endgroup]
2023-01-05T20:38:52.4254620Z Traceback (most recent call last):
2023-01-05T20:38:52.4271070Z   File "/Users/runner/hostedtoolcache/Python/3.7.15/x64/bin/flake8", line 8, in <module>
2023-01-05T20:38:52.4272280Z     sys.exit(main())
2023-01-05T20:38:52.4274130Z   File "/Users/runner/hostedtoolcache/Python/3.7.15/x64/lib/python3.7/site-packages/flake8/main/cli.py", line 22, in main
2023-01-05T20:38:52.4274960Z     app.run(argv)
2023-01-05T20:38:52.4276040Z   File "/Users/runner/hostedtoolcache/Python/3.7.15/x64/lib/python3.7/site-packages/flake8/main/application.py", line 363, in run
2023-01-05T20:38:52.4277030Z     self._run(argv)
2023-01-05T20:38:52.4278120Z   File "/Users/runner/hostedtoolcache/Python/3.7.15/x64/lib/python3.7/site-packages/flake8/main/application.py", line 350, in _run
2023-01-05T20:38:52.4279000Z     self.initialize(argv)
2023-01-05T20:38:52.4281040Z   File "/Users/runner/hostedtoolcache/Python/3.7.15/x64/lib/python3.7/site-packages/flake8/main/application.py", line 330, in initialize
2023-01-05T20:38:52.4282070Z     self.find_plugins(config_finder)
2023-01-05T20:38:52.4283180Z   File "/Users/runner/hostedtoolcache/Python/3.7.15/x64/lib/python3.7/site-packages/flake8/main/application.py", line 153, in find_plugins
2023-01-05T20:38:52.4284120Z     self.check_plugins = plugin_manager.Checkers(local_plugins.extension)
2023-01-05T20:38:52.4285260Z   File "/Users/runner/hostedtoolcache/Python/3.7.15/x64/lib/python3.7/site-packages/flake8/plugins/manager.py", line 357, in __init__
2023-01-05T20:38:52.4286230Z     self.namespace, local_plugins=local_plugins
2023-01-05T20:38:52.4287680Z   File "/Users/runner/hostedtoolcache/Python/3.7.15/x64/lib/python3.7/site-packages/flake8/plugins/manager.py", line 238, in __init__
2023-01-05T20:38:52.4288530Z     self._load_entrypoint_plugins()
2023-01-05T20:38:52.4289700Z   File "/Users/runner/hostedtoolcache/Python/3.7.15/x64/lib/python3.7/site-packages/flake8/plugins/manager.py", line 254, in _load_entrypoint_plugins
2023-01-05T20:38:52.4291650Z     eps = importlib_metadata.entry_points().get(self.namespace, ())
2023-01-05T20:38:52.4292630Z AttributeError: 'EntryPoints' object has no attribute 'get'
2023-01-05T20:38:52.4429730Z ##[error]Process completed with exit code 1.

shrug

[bemoody]

Related issue? https://github.com/PyCQA/flake8/issues/1701