dev: Ignore deprecation warnings generated by pytest 7 for pytest-flake8

The warning was turned into an error by our pytest config, and thus
resulted in test failures.

There's an existing issue for pytest-flake8¹ that's 3 months old now
with no movement so far.  setuptools is also dealing with the same
warnings.²

If other issues arise that aren't fixed upstream, a longer term solution
could be doing what we do with mypy and simply invoke flake8 directly
instead of via a pytest plugin.

¹ tholo/pytest-flake8#83
² pypa/setuptools#3079

pytest.ini

@@ -13,3 +13,5 @@ python_functions = pytest_*
 filterwarnings =
     error
     ignore:SelectableGroups dict interface:DeprecationWarning:flake8
+    ignore:<class 'pytest_flake8[.]Flake8Item'> is not using a cooperative constructor:pytest.PytestDeprecationWarning
+    ignore:The .+? argument to Flake8Item is deprecated:pytest.PytestRemovedIn8Warning