release-20.2:Backport!4240

Merged Andrey Vertiprahov requested to merge cherry-pick-f40d1e54-2 into release-20.2

Ubiquiti.AirOS.get_version. Cleanup for py3.

See merge request !4240 (merged)

(cherry picked from commit f40d1e54)

27238771 Ubiquiti.AirOS.get_version. Cleanup for py3.

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