Type get_response with a callback protocol.

Otherwise, calling `self.get_response(request)` in a subclass of
`MiddlewareMixin` runs into `Invalid self argument` error.

This is a workaround for python/mypy#5485.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

django-stubs/utils/deprecation.pyi

@@ -27,7 +27,8 @@ class DeprecationInstanceCheck(type):
     deprecation_warning: Type[Warning]
     def __instancecheck__(self, instance: Any): ...
 
-GetResponseCallable = Callable[[HttpRequest], HttpResponse]
+class GetResponseCallable:
+    def __call__(request: HttpRequest) -> HttpResponse: ...
 
 class MiddlewareMixin:
     get_response: GetResponseCallable = ...