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

component/pygolang/test-py3.cfg

+# SlapOS software release to test pygolang/py3 on Nexedi testing infrastructure.
+
+[buildout]
+extends = test.cfg
+
+[python]
+part = python3

stack/slapos.cfg

@@ -164,6 +164,7 @@ dateparser = 0.7.6
 decorator = 4.3.0
 funcsigs = 1.0.2
 gevent = 20.9.0
+geventmp = 0.0.1
 greenlet = 0.4.17
 idna = 2.9
 importlib-metadata = 1.7.0