Make decorator functions transparent to Mypy (#306)

By declaring return type as -> Callable[[_C], _C], Mypy can infer that
the decorated function has also the same arguments and return type as
the original.

View functions are constrained to return HttpResponseBase (or any
subclass of it).

Also added typecheck test coverage to most of the cases.

test-data/typecheck/contrib/auth/test_decorators.yml

@@ -0,0 +1,43 @@
+-   case: login_required_bare
+    main: |
+        from django.contrib.auth.decorators import login_required
+        @login_required
+        def view_func(request): ...
+        reveal_type(view_func)  # N: Revealed type is 'def (request: Any) -> Any'
+-   case: login_required_fancy
+    main: |
+        from django.contrib.auth.decorators import login_required
+        from django.core.handlers.wsgi import WSGIRequest
+        from django.http import HttpResponse
+        @login_required(redirect_field_name='a', login_url='b')
+        def view_func(request: WSGIRequest, arg: str) -> HttpResponse: ...
+        reveal_type(view_func)  # N: Revealed type is 'def (request: django.core.handlers.wsgi.WSGIRequest, arg: builtins.str) -> django.http.response.HttpResponse'
+-   case: login_required_weird
+    main: |
+        from django.contrib.auth.decorators import login_required
+        # This is non-conventional usage, but covered in Django tests, so we allow it.
+        def view_func(request): ...
+        wrapped_view = login_required(view_func, redirect_field_name='a', login_url='b')
+        reveal_type(wrapped_view)  # N: Revealed type is 'def (request: Any) -> Any'
+-   case: login_required_incorrect_return
+    main: |
+        from django.contrib.auth.decorators import login_required
+        @login_required()  # E: Value of type variable "_VIEW" of function cannot be "Callable[[Any], str]"
+        def view_func2(request) -> str: ...
+-   case: user_passes_test
+    main: |
+        from django.contrib.auth.decorators import user_passes_test
+        @user_passes_test(lambda u: u.username.startswith('super'))
+        def view_func(request): ...
+        reveal_type(view_func)  # N: Revealed type is 'def (request: Any) -> Any'
+-   case: user_passes_test_bare_is_error
+    main: |
+        from django.http.response import HttpResponse
+        from django.contrib.auth.decorators import user_passes_test
+        @user_passes_test  # E: Argument 1 to "user_passes_test" has incompatible type "Callable[[Any], HttpResponse]"; expected "Callable[[AbstractUser], bool]"
+        def view_func(request) -> HttpResponse: ...
+-   case: permission_required
+    main: |
+        from django.contrib.auth.decorators import permission_required
+        @permission_required('polls.can_vote')
+        def view_func(request): ...