requirements-test: Pin pytest to < 7.0.0

Related to T3916

requirements-test.txt

 django-stubs
 djangorestframework-stubs
-pytest
+pytest < 7.0.0  # v7.0.0 removed _pytest.tmpdir.TempdirFactory, which is used by some of the pytest plugins we use
 pytest-django
 pytest-mock
 requests_mock