[docs] [issue33187] Document ElementInclude (XInclude) support in ElementTree

[Stefan Behnel]

Change by Stefan Behnel <stefan_ml at behnel.de>:


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versions: +Python 3.4, Python 3.5, Python 3.6, Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33187>
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