fix: remove SuccessMessageMixin.form_valid violation of Liskov substitution principle (typeddjango#84)

Author: antonagestam

django-stubs/contrib/messages/views.pyi

@@ -1,9 +1,9 @@
 from typing import Any, Dict, Optional
 
-from django.forms.forms import Form
+from django.forms.forms import BaseForm
 from django.http.response import HttpResponseRedirect
 
 class SuccessMessageMixin:
     success_message: str = ...
-    def form_valid(self, form: Form) -> HttpResponseRedirect: ...
+    def form_valid(self, form: BaseForm) -> HttpResponseRedirect: ...
     def get_success_message(self, cleaned_data: Dict[str, str]) -> str: ...