Update AppVeyor configuration to drop Python 2.6 support - #4450

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What about testing against 3.5 {rather than}/{and} 3.4?

+1 for testing Python 3.5.

Agreed, might as well test python 3.5

I've added Python 3.5, but should I remove 3.4? It looks like it takes 10 minutes per configuration, so it's now going to take around 30 minutes per build. Maybe that's ok? (I've noticed appveyor finishing before Travis these days)

@astrofrog - I would remove 3.4; we're testing it on linux, so it really should be OK, and even if it doesn't save time, it does leave resources for others (and saves some little bit of power).

@embray - it looks like there are some FITS-related failures on Windows with Python 3.5 - any idea what those are due to?

@astrofrog I believe all of them are due to #4342

@embray @mhvk @bsipocz - I've left 3.5 in as an allowed failure. Since this runs faster than Travis, I've left in 3.4 otherwise we have no indicator that 3.x is failing. Once 3.5 is unmarked as an allowed failure, we can remove 3.4.

Maybe make a note in #4342 that that should be done as part of a fix for that issue.

I'll go ahead and merge this, and I've made a note in #4342