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Fix the issues with entvalidate.py

Description

Fix the issues with entvalidate.py

It doesn't properly handle mixins / special templates
On python 3.8+, XML elements are no longer sorted and this fails validation against entity.rng
mixins are not properly applied because the | split is incorrect (3 max items instead of 2)
merge="" is not supported
Parents aren't always passed when they should be.
on windows it is incorrectly assumed that the encoding is 1252

Patch by: @baco
Tweaks by: @Stan
Based on a patch by: @wraitii

Differential Revision: https://code.wildfiregames.com/D4698