Add default __getattr__ behavior to models.TokenUser (#528)

* Add default __getattr__ behavior to models.TokenUser to allow getting custom claims defined in serializers

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

rest_framework_simplejwt/models.py

@@ -103,3 +103,7 @@ def is_authenticated(self):
 
     def get_username(self):
         return self.username
+
+    def __getattr__(self, attr):
+        """This acts as a backup attribute getter for custom claims defined in Token serializers."""
+        return self.token.get(attr, None)