Fixes for Python 3.9
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.