refactor(config/base_config): make set_key not implemented

it's not used in base_config but should be overwritten in a child class

Author: Lee-W

commitizen/config/base_config.py

@@ -23,7 +23,7 @@ def set_key(self, key, value):
         For now only strings are supported.
         We use to update the version number.
         """
-        return self
+        raise NotImplementedError()
 
     def update(self, data: dict):
         self._settings.update(data)