v0.8.0

• Removed one-time token behavior from jwt.Credentials, audience claim is now required and fixed. (#117)
• crypt.Signer and crypt.Verifier are now abstract base classes. The concrete implementations have been renamed to crypt.RSASigner and crypt.RSAVerifier. app_engine.Signer and iam.Signer now inherit from crypt.Signer. (#115)
• transport.grpc now correctly calls Credentials.before_request. (#116)