Avoid cElementTree on Python 3

[uranusjr]

cElementTree has been deprecated since 3.3, and is simply an alias to ElementTree right now. It will be removed in the Python 3.10.

Originally filed as: pypa/pip#8278
See also: python/cpython#19921

[gsnedders]

What prompts the desire to avoid it? Avoiding the PendingDeprecationWarning proposed to be added in python/cpython#19921 but not python/cpython#20117? This this causes warnings with the perfectly sane try/except ImportError that's so prevalent and future-proof seems ridiculous.

[uranusjr]

cc-ing @tiran since I do not have the knowledge to answer the question myself.

[tiran]

The removal of cElementTree without a deadline and deprecation warning broke a bunch of packages. I like to re-introduce the module in 3.9 with a warning and remove it in 3.10.

The warning is breaking Python's ensurepip tests. Pip depends on html5lib and html5lib triggers the warning. My patch avoids the warning and keeps the behavior of html5lib. If you want a simpler patch, then just import xml.etree.ElementTree in all Python versions. This works in Python 2, but it's going to slow down parsing on Python 2 a bit.