dev: Ignore a deprecation warning generated by flake8's usage of impo…

…rtlib_metadata

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

importlib_metadata used to specially suppress this deprecation warning
for flake8, but that special-casing was removed in June without any
update to flake8's to avoid the warning.  See

    python/importlib_metadata#319

and the flake8 PRs it links to for more background.

pytest.ini

@@ -9,6 +9,7 @@ python_functions = pytest_*
 
 # Turn warnings into errors.  Warnings are signs that something needs
 # attention, even if that means choosing to add an explicit ignore filter rule
-# for them.
+# for them.  Last matching filter wins.
 filterwarnings =
     error
+    ignore:SelectableGroups dict interface:DeprecationWarning:flake8