Add a --parallel flag for build_docs

[astrofrog]

This does several things:

• Adds a --parallel option for build_docs
• Drops support for Python 3.5
• Adds a requirement on sphinx-astropy 1.2 - this is a very recent release, but I think probably ok since this is mostly for package maintainers/developers who can easily update?
• Requiring sphinx-astropy 1.2 also allows us to support only Sphinx 1.7 and above, so simplifies some of the logic here
• Removed leftover references to auto-installing sphinx-astropy
• Switch to Travis CI for Windows builds (I couldn't fix the AppVeyor one)

[astrofrog]

Trying to debug the failing CI...

[codecov]

Codecov Report

Merging #498 into master will increase coverage by 0.58%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master     #498      +/-   ##
==========================================
+ Coverage   40.96%   41.54%   +0.58%     
==========================================
  Files          19       19              
  Lines        1665     1661       -4     
==========================================
+ Hits          682      690       +8     
+ Misses        983      971      -12

Impacted Files	Coverage Δ
astropy_helpers/tests/test_setup_helpers.py	0% <0%> (ø)	⬆️
astropy_helpers/commands/build_sphinx.py	78.18% <72.72%> (+1.45%)	⬆️
astropy_helpers/setup_helpers.py	66.78% <0%> (+0.7%)	⬆️
astropy_helpers/utils.py	60.97% <0%> (+7.31%)	⬆️

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[bsipocz]

some comments @astrofrog, but nothing drastic

[bsipocz]

what do we install from conda that we need from the astropy channel?

[bsipocz]

e.g. could just add this to the one build that conda installs sphinx-astropy rather than being a global setting (probably doesn't matter much, but I expect things might be slightly be faster using just the default channels)

[astrofrog]

Done

[bsipocz]

we need python 3.8 builds, too

[astrofrog]

I think we can do that in a separate PR since that's well beyond the scope of this one :)

[bsipocz]

would also mention the change in the minimum sphinx version

[astrofrog]

Done

[bsipocz]

probably worth mentioning in the changelog that there is no more auto installing

[astrofrog]

This was actually done in #474 for 3.2 - I'm just removing leftover comments.

[bsipocz]

Thanks @astrofrog!