net/py-raet: Update USES=python to include python3

Python3 support was added in raet version 0.5.0:
https://github.com/RaetProtocol/raet/blob/master/ChangeLog.md#20150204-2

Exactly which python 3 versions are supported / known to work will be
discussed here:
https://github.com/RaetProtocol/raet/issues/12

We know at least python 3.5 and 3.6 are supported:
https://github.com/RaetProtocol/raet/commit/19391b599cdde2399889522c442f6bd779525ce3
https://github.com/RaetProtocol/raet/commit/16a05b81daaaa8ec7933719e312b5f0274ee3ade

PR: 244653, 243610
Submitted by: Tommy P <tommyhp2@gmail.com>
Approved by: portmgr (blanket: dependency fix, ports (python) compliance)
MFH: 2020Q1