Reflect the deprecation of get_response being None. (#1086)

* Reflect the deprecation of get_response being None.

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

* 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>

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

django-stubs/middleware/cache.pyi

@@ -27,7 +27,7 @@ class CacheMiddleware(UpdateCacheMiddleware, FetchFromCacheMiddleware):
     cache: BaseCache = ...
     def __init__(
         self,
-        get_response: Optional[Callable] = ...,
+        get_response: Callable = ...,
         cache_timeout: Optional[float] = ...,
         page_timeout: Optional[float] = ...,
         **kwargs: Any

django-stubs/utils/deprecation.pyi

@@ -1,4 +1,4 @@
-from typing import Any, Callable, Optional, Type
+from typing import Any, Callable, Optional, Protocol, Type
 
 from django.http.request import HttpRequest
 from django.http.response import HttpResponse
@@ -27,9 +27,10 @@ class DeprecationInstanceCheck(type):
     deprecation_warning: Type[Warning]
     def __instancecheck__(self, instance: Any): ...
 
-GetResponseCallable = Callable[[HttpRequest], HttpResponse]
+class GetResponseCallable(Protocol):
+    def __call__(self, __request: HttpRequest) -> HttpResponse: ...
 
 class MiddlewareMixin:
-    get_response: Optional[GetResponseCallable] = ...
-    def __init__(self, get_response: Optional[GetResponseCallable] = ...) -> None: ...
+    get_response: GetResponseCallable = ...
+    def __init__(self, get_response: GetResponseCallable = ...) -> None: ...
     def __call__(self, request: HttpRequest) -> HttpResponse: ...