python 3.9 compatibility

cElementTree is deprecated since 3.3 and has been removed in 3.9 [0]

Since 3.3.0, cElementTree is a dumb alias to ElementTree [1].

First reported in Gentoo [2]

• [0] https://docs.python.org/3.9/whatsnew/3.9.html#removed
• [1] https://github.com/python/cpython/blob/v3.3.0/Lib/xml/etree/cElementTree.py
• [2] https://bugs.gentoo.org/726852