No longer compatible with pytest 7

[jaraco]

See pytest-dev/pytest#8435 for background and jaraco/pytest-checkdocs#9 for an example of how I addressed the issue in pytest-checkdocs.

[jaraco]

See also jaraco/pytest-perf#5 for another, different fix that was required.

[abravalheri]

The following waning/error also appears for 7.0.1:

pytest.PytestWarning: BlackItem is an Item subclass and should not be a collector, however its bases File are collectors.
E   Please split the Collectors and the Item into separate node types.
E   Pytest Doc example: https://docs.pytest.org/en/latest/example/nonpython.html
E   example pull request on a plugin: https://github.com/asmeurer/pytest-flakes/pull/40/