Commit 8af78fc5 (pyx.build: v↑ setuptools_dso (1.2 -> 1.4)) upgraded
setuptools_dso to 1.4, but since from

https://github.com/mdavidsaver/setuptools_dso/commit/3f3ff746

setuptools_dso started to use multiprocessing, pyx.build, when running
under gpython, started to hang, which is a known gevent problem - see
e.g. here: https://github.com/gevent/gevent/issues/993. The problem was
manifesting itself as pyx.build unit test hanging under Python3.

Fix it by installing gevent multiprocessing plugin which is
automatically used/activated by gevent.monkey.patch_all().

geventmp says it is pre-alpha, but by using it we can unhang pyx.build
tests, which is better state than before. The other future possibility
would be to use https://github.com/jgehrcke/gipc wrapped into
multiprocessing compatible API.