RF: use python not python3 for running sphinx-build

[yarikoptic]

To resolve a hypothetical issue of no python3 executable being available, while still allowing for virtualenv and system-wide installation of sphinx.

I will merge if does not cause any breakage in CI

[codecov[bot]]

Codecov Report

Merging #491 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #491   +/-   ##
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  Coverage   89.64%   89.64%           
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  Files         148      148           
  Lines       12193    12193           
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  Hits        10931    10931           
  Misses       1262     1262

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[kyleam]

In https://github.com/ReproNim/reproman/pull/485#discussion_r352753782 you said

once again, I think it would be better to just change it to python. Having said that, do you know a distribution where having any version of python3 installed would result in no python3 executable?

I don't. I mistakenly thought arch had only a python (which is py3), but it's just that arch uses python 3 for their plain python executable, going against pep-0304. arch still has a python3 executable. Given pep-0304, I'd say assuming the presence of a python3 executable is correct, and my concerns from gh-485 that prompted this PR are misguided. So I'd vote to close this without a merge.

[yarikoptic]

d'oh, saw your comment only upon refreshing the page and after the merge. One way or another I think it all should work, so let's let it RiP until we actually hit an issue