Add minimum version requirement for pytest

Using older versions works for passing tests, but assertion failures
cause a very obscure traceback on teardown

requirements-test.txt

@@ -5,7 +5,7 @@ django-test-migrations
 hypothesis
 # TODO: remove that pin once we bump mypy to > 0.950 in tox.ini
 mypy == 0.942
-pytest
+pytest >= 7.0.0 # older versions work too, but assertion failures cause https://github.com/untitaker/pytest-subtesthack/issues/4
 pytest-django
 pytest-mock
 pytest-postgresql < 4.0.0