I chose to only add W_STOPCODE() to _testcapi rather than the os module, because I don't want to have to document this function. I don't think that anyone needs such function, usually we only need to consume process statuses, not to produce them. The only use case is to write an unit test.

This issue is part of bpo-31160 which ensures that unit tests don't leak child processes. This issue is part of my large project of reducing the fail rate on CIs (Travis CI, AppVeyor, buildbots):
https://haypo.github.io/python-buildbots-2017q2.html

I will now merge my PR 3055 to be able to unblock my work on CIs. But I will wait for feedback from Gregory before backporting this fix to 2.7 and 3.6.