"language: python" should be supported on "os: windows" and "os: osx"

On os: windows and os: osx the tag language: python should be allowed even if it does nothing at all.

On os: osx the tag language: generic works but it does not work on os: windows.

The best would be if language: python did a brew install python on os: osx and a choco install python on os: windows because the current approach is too inconsistent and error-prone.

With just 345 days until Python 2 end of life, Travis CI should stop defaulting to legacy Python… http://pythonclock.org