Revert "requirements-test: Temporarily pin django-stubs to < 1.10.0"

This reverts commit 4083f796.

requirements-test.txt

-django-stubs < 1.10.0
+django-stubs
 djangorestframework-stubs
 pytest < 7.0.0  # v7.0.0 removed _pytest.tmpdir.TempdirFactory, which is used by some of the pytest plugins we use
 pytest-django