Fix manager types scope (#991)

* Fix manager types scope

* Restore incremental mode and mention in developer docs

* Separate dev mypy config and regular one

* Document config files usage

pytest.ini

@@ -7,5 +7,5 @@ addopts =
     -s
     -v
     --cache-clear
-    --mypy-ini-file=./mypy.ini
+    --mypy-ini-file=./mypy.ini.dev
     --mypy-extension-hook=scripts.tests_extension_hook.django_plugin_hook