Allow mutable extra_context on TemplateViews

Prior to this change, the `extra_context` attribute of a template view
was annotated as a `dict` (or `None`). This means that if a value is
provided, it *must* be a mutable object. Since this is an attribute on
the class, this creates mutable shared state, which is a source of bugs.
It was not possible to use an immutable mapping (`MappingProxyType or
the 3rd-party `frozendict`) without getting type-checking errors.

However, there is no need for this attribute to be mutable. This change
allows *any* mapping type, which allows authors to write template views
with immutable `extra_context` attributes.

Refs #1992

django-stubs/views/generic/base.pyi

@@ -1,6 +1,6 @@
 import logging
 from collections.abc import Callable, Sequence
-from typing import Any
+from typing import Any, Mapping
 
 from django.http.request import HttpRequest
 from django.http.response import HttpResponse, HttpResponseBase
@@ -9,7 +9,7 @@ from django.utils.functional import _Getter
 logger: logging.Logger
 
 class ContextMixin:
-    extra_context: dict[str, Any] | None
+    extra_context: Mapping[str, Any] | None
     def get_context_data(self, **kwargs: Any) -> dict[str, Any]: ...
 
 class View:

tests/typecheck/views/generic/test_template.yml

@@ -0,0 +1,16 @@
+-   case: generic_template_view_mutable_extra_context
+    main: |
+        from django.views.generic import TemplateView
+
+        class MyTemplateView(TemplateView):
+            template_name = "template.html"
+            extra_context = {}
+
+-   case: generic_template_view_immutable_extra_context
+    main: |
+        from types import MappingProxyType
+        from django.views.generic import TemplateView
+
+        class MyTemplateView(TemplateView):
+            template_name = "template.html"
+            extra_context = MappingProxyType({})