Factor out shell logic from tox.ini

[bsipocz]

And also add calling make html instead of sphinx-build

[danielballan]

FWIW I am in favor of tox calling sphinx directly: I think task-runners calling other task-runners might obfuscate the action in way that is confusing for readers.

[bsipocz]

I very strongly feel that all the files in the repo should be called by a runner otherwise the chances that they bit rot is non-negligible. I don't particularly care how to achieve that, I don't feel bad about calling make html instead of calling the same sphinx line.

[danielballan]

all the files in the repo should be called by a runner

Fully agree.

I don't feel bad about calling make html [from inside tox.ini]

OK, I don't feel strongly about this.