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

mypy.ini.dev

@@ -0,0 +1,30 @@
+# Configuration file to use during development
+
+# Changes of plugin code are not detected by mypy, so
+# incremental mode can be harmful during development
+# (mypy cache is not always invalidated).
+# However, incremental mode has to be supported by django-stubs,
+# thus another config (mypy.ini) is used in CI.
+# Incremental mode is recommended for regular usage.
+
+# WARNING: when changing this file, consider doing the same with mypy.ini
+
+[mypy]
+allow_redefinition = True
+check_untyped_defs = True
+ignore_missing_imports = True
+# Avoid caching between test runs
+incremental = False
+strict_optional = True
+show_traceback = True
+warn_no_return = False
+warn_unused_ignores = True
+warn_redundant_casts = True
+warn_unused_configs = True
+warn_unreachable = True
+
+plugins =
+    mypy_django_plugin.main
+
+[mypy.plugins.django-stubs]
+django_settings_module = scripts.django_tests_settings