[9.20] chg: test: Temporarily change pytest.PytestRemovedIn9Warning error to warning

We need this to be able to use pytest 9.0 from Debian "sid".

Additionally, this error needs to be addressed before pytest 9.1 is
released, as the filter will no longer work. Also, all pytests in CI
images need to be upgraded to version 7, where the new API is supported.

Related #5690

Backport of MR !11379

Merge branch 'backport-mnowak/ignore-pytest-PytestRemovedIn9Warning-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11380