Update i18n scripts to run on Python3

The pology library runs on Python2 and development appears stalled. It is also not available on pip.
The babel library, BSD-licensed, provides, amongst many other things, a replacement for .POT / .PO manipulation.

The poediff tool that we used to detect spurious i18n change is replaced with a Python script that does a simpler but good enough job (it is also much, much faster).

These replacements let the i18n scripts run on Python3 entirely.

Makes D506 redundant.

Comments by: Itms

Reviewed By: Gallaecio

Refs #5694

Differential Revision: https://code.wildfiregames.com/D2757