pygolang: Fix build on Python3

Jérome reports that Pygolang build on Python3 fails because geventmp turned out to be not pinned:

https://lab.nexedi.com/nexedi/slapos/merge_requests/862#note_121359

Geventmp is indeed used by pygolang only on Python3

https://lab.nexedi.com/nexedi/pygolang/commit/cd67996e

and with Python2-only testing I missed to handle it.

-> Fix it and add test for pygolang/py3.

/cc @jerome