rewrite pytest-mypy-plugin extension code to use new hook

pytest.ini

@@ -2,7 +2,8 @@
 testpaths = ./test-data
 addopts =
     --tb=native
-    --mypy-ini-file=./test-data/plugins.ini
     -s
     -v
-    --cache-clear
+    --cache-clear
+    --mypy-ini-file=./test-data/plugins.ini
+    --mypy-extension-hook=scripts.tests_extension_hook.django_plugin_hook