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/utils/test_decorators.yml

@@ -0,0 +1,20 @@
+-   case: method_decorator_class
+    main: |
+      from django.views.generic.base import View
+      from django.utils.decorators import method_decorator
+      from django.contrib.auth.decorators import login_required
+      @method_decorator(login_required, name='dispatch')
+      class TestView(View): ...
+      reveal_type(TestView())  # N: Revealed type is 'main.TestView'
+-   case: method_decorator_function
+    main: |
+      from django.views.generic.base import View
+      from django.utils.decorators import method_decorator
+      from django.contrib.auth.decorators import login_required
+      from django.http.response import HttpResponse
+      from django.http.request import HttpRequest
+      class TestView(View):
+          @method_decorator(login_required)
+          def dispatch(self, request: HttpRequest, *args, **kwargs) -> HttpResponse:
+              return super().dispatch(request, *args, **kwargs)
+          reveal_type(dispatch)  # N: Revealed type is 'def (self: main.TestView, request: django.http.request.HttpRequest, *args: Any, **kwargs: Any) -> django.http.response.HttpResponse'