1.13.1 breaks pymunk on linux in a subtle way

Created originally on Bitbucket by viblo (Victor Blomqvist)

It seems like something in the latest release (1.13.1) breaks some things in the python library pymunk as described in this github issue: https://github.com/viblo/pymunk/issues/171

I have tried to debug, and it seems to be something that happens on Linux (tested on ubuntu) on CPython. On Windows it works as expected. I tried readin the changelog for 1.13.1 but could not see any obvious reasons why it would stop working or break in the way it does.