Replace str with _StrOrPromise where the latter is expected.

Fixes typeddjango#1231

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

django-stubs/contrib/messages/views.pyi

@@ -2,8 +2,9 @@ from typing import Dict
 
 from django.forms.forms import BaseForm
 from django.http.response import HttpResponse
+from django.utils.functional import _StrOrPromise
 
 class SuccessMessageMixin:
-    success_message: str = ...
+    success_message: _StrOrPromise = ...
     def form_valid(self, form: BaseForm) -> HttpResponse: ...
     def get_success_message(self, cleaned_data: Dict[str, str]) -> str: ...