Fix staff_member_required annotations (typeddjango#436)

* Fix staff_member_required annotations

`redirect_field_name` can be `None`.

* Reformat code with black

django-stubs/contrib/admin/views/decorators.pyi

@@ -1,7 +1,11 @@
-from typing import Callable, TypeVar, overload
+from typing import Callable, Optional, TypeVar, overload
 
 _C = TypeVar("_C", bound=Callable)
 @overload
-def staff_member_required(view_func: _C = ..., redirect_field_name: str = ..., login_url: str = ...) -> _C: ...
+def staff_member_required(
+    view_func: _C = ..., redirect_field_name: Optional[str] = ..., login_url: str = ...
+) -> _C: ...
 @overload
-def staff_member_required(view_func: None = ..., redirect_field_name: str = ..., login_url: str = ...) -> Callable: ...
+def staff_member_required(
+    view_func: None = ..., redirect_field_name: Optional[str] = ..., login_url: str = ...
+) -> Callable: ...