Disable fallback to runtime types for Django settings (#1163)

This fallback to value.__class__ seems to be doing more harm than
good; see #312 and #1162.  Replace it with a clear error message that
suggests a way to fix the problem rather than incompletely papering
over it.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

mypy_django_plugin/django/context.py

@@ -62,7 +62,7 @@ def initialize_django(settings_module: str) -> Tuple["Apps", "LazySettings"]:
         apps.get_swappable_settings_name.cache_clear()  # type: ignore
 
         if not settings.configured:
-            settings._setup()
+            settings._setup()  # type: ignore
 
         apps.populate(settings.INSTALLED_APPS)