Log Message:
py-jwcrypto: add version 1.3.1

An implementation of the JOSE Working Group documents:
* RFC 7515 - JSON Web Signature (JWS)
* RFC 7516 - JSON Web Encryption (JWE)
* RFC 7517 - JSON Web Key (JWK)
* RFC 7518 - JSON Web Algorithms (JWA)
* RFC 7519 - JSON Web Token (JWT)
* RFC 7520 - Examples of Protecting Content Using JSON Object Signing
             and Encryption (JOSE)