pygolang: Fix build on Python3

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

!862 (comment 121359)

Geventmp is indeed used by pygolang only on Python3

pygolang@cd67996e

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

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

/reviewed-on !864