The problem (I think) is that default compiler on Windows changed in python 3.5 but the appveyor setup scripts (%CMD_IN_ENV% in particular) configures the compiler to that used in python 3 prior to 3.5.

Will end up resolving this over the weekend for the affiliate builder, I hope, and port the change here (or to the new CI helper package).

@mwcraig - it would be great to fix this in the ci-helpers if this applies to all packages with compiled code

@astrofrog - And also we have to come up with something to reproduce this error in ci-helpers, as it seems that the py3.5 builds pass there.

@bsipocz -- actually, I don't think it is necessary to add a test. Changing the default compiler seems to happen only every ~5+ years, and my understanding is that in the future it will not be necessary to match compiler versions as is necessary now, so the current config should, in principle, be good for a long while. I don't understand all of it, but the current situation is discussed here: http://stevedower.id.au/blog/building-for-python-3-5-part-two/

Fixed in #296