Make py2 and py3 aliases also resolve via py on windows

[asottile]

Resolves #856

CC @pfmoore

[gaborbernat]

Take a look at the ci as it broke on UNIX.

[asottile]

heh, this was passing for the wrong reason before (the assertion succeeded because the exit code of check_call was never verified) -- I guess I can add a similar -c '' check for unix as well

[codecov]

Codecov Report

Merging #858 into master will increase coverage by <1%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #858    +/-   ##
======================================
+ Coverage      92%    92%   +<1%     
======================================
  Files          13     13            
  Lines        2421   2421            
  Branches      430    430            
======================================
+ Hits         2221   2224     +3     
+ Misses        124    123     -1     
+ Partials       76     74     -2

[asottile]

@gaborbernat should be reviewable now!

[gaborbernat]

@asottile py2 and py3 work as aliases for the basepython property, not? If that's the case we should document this under the basepython config.
Furthermore, I'm not sure if this is what we want, as this would make our config not cross-platform. I mean when will I pass in py2?

Or is this only so that I can do tox -e py2 which will pick up whatever python 2 I have on windows and run the default testenv?

[asottile]

This enables the following things:

• tox -e py2 + tox -e py3 to work on windows (they already work on posix)
• basepython=python2 / basepython=python3 to work on windows (they already work on posix, and basepython=pythonX.X already works on windows)

This does not enable anything (imo) nonstandard like basepython=py2

[pfmoore]

Thanks for this! Sorry I didn't have time to test it - you guys are to fast for me 😄

[gaborbernat]

We plan to do a release with 3.1 next week together with #852. If you find any issues with that feel free to come back and fill a bug.