[issue40064] py38: document xml.etree.cElementTree will be removed in 3.9

[Chitrank-Dixit]

New submission from Chitrank-Dixit <chitrankdixit at gmail.com>:

Looking at this issue, I can see the changes in the file `xml.etree.elementtree` should go like

.. warning::

   The :mod:`xml.etree.ElementTree` will be removed from python 3.9.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40064>
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