Fixes for Python 3.9

Merged Björn Esser requested to merge besser82/xcbproto:topic/besser82/python39 into master

xcbgen: Use math.gcd() for Python >= 3.5.

fractions.gcd() has been deprecated since Python 3.5, and was finally dropped in Python 3.9. It is recommended to use math.gcd() instead.

xcbgen: xml.etree.cElementTree has been dropped in Python 3.9.

It can be replaced with xml.etree.ElementTree safely.

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