Fix JSONWebSignatureSerializer.load_dangerous() signature to satisfy …

…mypy. (#1135)

The latest mypy complained that the signature didn't match that in the
superclass: Serializer.load_payload() has a serializer argument.  I
don't know this project but I think it's best to just add that
argument (rather than adding `# type: ignore`).

third_party/3/itsdangerous.pyi

@@ -128,7 +128,7 @@ class JSONWebSignatureSerializer(Serializer):
     algorithm_name = ...  # type: str
     algorithm = ...  # type: Any
     def __init__(self, secret_key: _can_become_bytes, salt: Optional[_can_become_bytes]=None, serializer: _serializer=None, signer: Optional[Callable[..., Signer]]=None, signer_kwargs: Optional[MutableMapping]=None, algorithm_name: Optional[str]=None) -> None: ...
-    def load_payload(self, payload: Any, return_header: bool=False) -> Any: ...
+    def load_payload(self, payload: Any, serializer: _serializer = None, return_header: bool=False) -> Any: ...
     def dump_payload(self, *args, **kwargs) -> bytes: ...
     def make_algorithm(self, algorithm_name: str) -> SigningAlgorithm: ...
     def make_signer(self, salt: Optional[_can_become_bytes]=None, algorithm_name: Optional[str]=None) -> Signer: ...