Remove xml.etree.ElementTree.Element.copy() (Python implementation): use copy.copy() instead

[vstinner]

The xml.etree.ElementTree.Element.copy() method (Python implementation) was deprecated in Python 3.10 by commit 7d952de: see issue #76605 for the rationale.

The method only exists in the pure Python implementation. The C implementation only has a __copy__() method.

Use copy.copy() instead.