[python-experimental] Raise builtin AttributeError instead of custom ApiKeyError

Created by: sebastien-rosset

Currently when invoking the getattr, setattr, hasattr functions, the generated ApiKeyError exception is raised. This causes non-standard, non-compliant behavior.

This PR change the implementation (use AttributeError) to ensure compliance with the Python documentation:

1. The builtin getattr function has the following contract: If the named attribute does not exist, default is returned if provided, otherwise AttributeError is raised.
2. The builtin hasattr function returns True if the string is the name of one of the object’s attributes, False if not. (This is implemented by calling getattr(object, name) and seeing whether it raises an AttributeError or not.)

Similarly for the setattr function

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