requirements-test: Pin pytest to < 7.0.0

pytest v7.0.0 removed _pytest.tmpdir.TempdirFactory, which is used
by some of the pytest plugins we use.

requirements-test.txt

@@ -3,7 +3,7 @@ djangorestframework-stubs
 django-stubs
 django-test-migrations
 hypothesis
-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 != 1.9.0, != 1.9.1