Windows ARM32 worker on too many branches #90

Thanks to #89 we now have a Windows ARM32 buildbot.

Looking at https://buildbot.python.org/all/#/builders though, I see it's attached to 2.7, 3.6 and 3.7 branches, while the support is only coming in for 3.x:

Can/should we restrict the buildbot to only the 3.x branch? I assume it's just going to fail to build against the other branches.