Various Build Tweaks

This PR does a number of things:

* Tweak the (currently unused) Appveyor config.
* Update astropy_helpers
* Prevent the ANA extensions being built on Windows or under Python 3.

The build is currently failing on Appveyor due to a gzip bug of some kind. Appveyor is only enabled on my fork (not the main repo) so that should not prevent the merge of this PR.