requirements-test: Remove pytest pinning to < 7

pytest-postgresql 3.1.3 and pytest-redis 2.4.0 added support for
pytest >= 7 so we can now drop the pytest pinning.

requirements-test.txt

-pytest < 7.0.0  # v7.0.0 removed _pytest.tmpdir.TempdirFactory, which is used by some of the pytest plugins we use
+pytest
 pytest-django
 pytest-mock
 swh.scheduler[testing]