Fix undefined object in Python model __copy__ template

Created by: tuanchien

Implements the recommended fix for issue #10856 (closed), a Python client bug in the mustache template that causes model copies to fail due to trying to return an undefined object.

Changes the template for the __copy__ method to return cls.__new__(cls, **self.__dict__) instead of new_cls.__new__(cls, **self.__dict__)

Tagging Python committee: @taxpon @frol @mbohlool @cbornet @kenjones-cisco @tomplus @Jyhess @arun-nalla @spacether

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