fix: revise malformed __str__(self) function (#593)

Author: twilio-aiss

twilio/jwt/access_token/__init__.py

@@ -15,7 +15,7 @@ def to_payload(self):
         raise NotImplementedError('Grant must implement to_payload.')
 
     def __str__(self):
-        '<{} {}>'.format(self.__class__.__name__, self.to_payload())
+        return '<{} {}>'.format(self.__class__.__name__, self.to_payload())
 
 
 class AccessToken(Jwt):
@@ -69,4 +69,4 @@ def _generate_payload(self):
         return payload
 
     def __str__(self):
-        return '<AccessToken {}>'.format(self.to_jwt())
+        return '<{} {}>'.format(self.__class__.__name__, self.to_jwt())