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

There is an incompatibility between latest django-stubs release (1.10.0)
and current release of djangorestframework-stubs.

So temporarily pin django-stubs to < 1.10.0 until the issue is fixed.

requirements-test.txt

-django-stubs
+django-stubs < 1.10.0
 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