Serializer.load_payload overrides in subclasses have incompatible signatures

[ambv]

Mypy recently introduced stricter checks for function signature compatibility (python/mypy#2521). Since itsdangerous is included in the typeshed library with typing stubs, it found an issue with the signatures of load_payload. Consider:

class Serializer:
  def load_payload(self, payload, serializer=None): ...

vs.

class JSONWebSignatureSerializer(Serializer):
  def load_payload(self, payload, return_header=False): ...

vs.

class URLSafeSerializerMixin(object):
  def load_payload(self, payload): ...

class URLSafeSerializer(URLSafeSerializerMixin, Serializer): ...
class TimedSerializer(Serializer): ...
class URLSafeTimedSerializer(URLSafeSerializerMixin, TimedSerializer): ...

Basically, it's invalid to replace load_payload() accepting an optional kwarg with an implementation that doesn't.

[davidism]

PyCharm has complained about it for a while too. If you have a fix in mind, a patch would be great, otherwise I'll investigate.

[ambv]

Alright, PR ready.