Upgrade/pyjwt support

[monkut]

fix update in pyjwt where jwt.ExpiredSignature was changed to jwt.ExpiredSignatureError.

    jwt_payload = token_validator.validate(jwt_token)
  File ".../site-packages/django_cognito_jwt/validator.py", line 68, in validate
    except (jwt.InvalidTokenError, jwt.ExpiredSignature, jwt.DecodeError) as exc:
rest_framework.request.WrappedAttributeError: module 'jwt' has no attribute 'ExpiredSignature'

[Vektrat]

I can confirm this is broken, most likely since jpadilla/pyjwt@f690976 where they dropped deprecation support for the referenced error in this project

I think it'd also be a good idea to lock compatible versions in the requirements list

This PR would fix #34

[adamantike]

@mikedebock @mvantellingen, can you please review this change? The workaround in #34 is not an alternative if you want to use a newer version of libraries depending on pyjwt (e.g. latest versions of https://github.com/jazzband/djangorestframework-simplejwt require pyjwt>=2,<3)

[MichelML]

should be closed in favor of #41 (comment)

[mikedebock]

As mentioned by @MichelML this will be resolved in the upcoming 0.0.4 release.