xcbgen: Use xml.etree.ElementTree for Python >= 3.3.

In commit 7d58eed9 we started to use xml.etree.ElementTree for Python >= 3.9. In fact the xml.etree.cElementTree module has already been deprecated since Python 3.3.

Given this fact, we should start to use the xml.etree.ElementTree module beginning with Python 3.3.

See: https://github.com/python/cpython/commit/a72a98f24a19928e31dcc4cab2cd2ad0f1846e11

Edited by Björn Esser

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