Temporary workaround for pallets/werkzeug#1051

Starting with python/mypy#2521 mypy is performing stricter function signature
checks.

This makes the stubs diverge from the actual implementation but makes the stubs
internally consistent.  Since this is an actual typing issue in the base
implementation, we need to defer to the original authors to fix it.

third_party/2/werkzeug/datastructures.pyi

@@ -161,7 +161,7 @@ class Headers(Mapping):
     def __copy__(self): ...
 
 class ImmutableHeadersMixin:
-    def __delitem__(self, key, **kwargs): ...
+    def __delitem__(self, key, _index_operation=True): ...  # FIXME: This is invalid but works around https://github.com/pallets/werkzeug/issues/1051
     def __setitem__(self, key, value): ...
     set = ...  # type: Any
     def add(self, *args, **kwargs): ...