Misc build improvements (build docs & more)

[Oberon00]

No description provided.

[Oberon00]

For some reason, tox is not executing the docs enironment. It works locally (with the same tox version) and tox -vvvv shows no helpful output, so I'm absolutely clueless now how to fix this.

[lzchen]

For Travis to execute the docs environment, add the python version as a prefix in front of docs in the envlist

[lzchen]

Instead of having two settings for testenv (unless you want to run tox -e ...), you can take advantage of tox's conditional settings to make the file more brief.

[Oberon00]

But would it be shorter? In this case I thought putting it in an en extra environment would look nicer (since there are few things in common with other environments, and I guess we also don't want docs to execute for all different python versions).

[Oberon00]

For Travis to execute the docs environment, add the python version as a prefix in front of docs in the envlist

Ah, I see: https://tox-travis.readthedocs.io/en/stable/envlist.html
I'll adjust this.