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