Don't use cElementTree on Python 3
Don't use cElementTree on Python 3

It's been deprecated and will be removed in 3.9 or 3.10. 3.9.0b1 doesn't
have cElementTree. I'd like to bring it back with a deprecation warning
to drop in 3.10.

See: https://github.com/python/cpython/pull/19921
Signed-off-by: Christian Heimes <2314b2e3a4a1f7db165be2aafbf1efd78f28cc97@python.org>

pradyunsg/pip,pfmoore/pip,pradyunsg/pip,pypa/pip,sbidoul/pip,pfmoore/pip,pypa/pip,sbidoul/pip