AppVeyor tests fail

[MSeifert04]

The tests with AppVeyor fail because of:

error: The 'test' command requires the astropy package to be installed and importable.
Command exited with code 1

These failures started somewhere between 15 days ago (24.7.) and 22 days ago (17.7.) see AppVeyor history

cc @bsipocz @astrofrog

[bsipocz]

Thanks @MSeifert04, I've seen these and hoped to be able to fix them but this month in travel season for me and thus hasn't yet found the time.

[MSeifert04]

Could it have something to do with astropy/ci-helpers#128? That one was merged 17 days ago and seems like a potential candidate.