Temporary workaround for pallets/itsdangerous#74

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.

Author: Lukasz Langa

third_party/2/itsdangerous.pyi

@@ -143,7 +143,7 @@ class TimedJSONWebSignatureSerializer(JSONWebSignatureSerializer):
     def now(self) -> int: ...
 
 class URLSafeSerializerMixin:
-    def load_payload(self, payload: Any, **kwargs) -> Any: ...
+    def load_payload(self, payload: Any, serializer=None, return_header=False, **kwargs) -> Any: ...  # FIXME: This is invalid but works around https://github.com/pallets/itsdangerous/issues/74 
     def dump_payload(self, *args, **kwargs) -> str: ...
 
 class URLSafeSerializer(URLSafeSerializerMixin, Serializer):