Python search should prefer "python2" executable

[benesch]

On systems where Python 3 is the default, node-gyp complains about an incorrect Python version, even though Python 2 is often available as python2.

Per PEP 394:

Python 2 only scripts should either be updated to be source compatible with Python 3 or else to use python2 in the shebang line

There are a few distributions that don't provide a python2 executable. But searching PATH for both python2 and python (and preferring python2) should catch cases where Python 2 is installed but node-gyp selects the wrong executable, while still supporting said distributions that don't ship a python2.

Is this something you guys'd endorse? If so, happy to provide a pull request.

[tjanson]

I agree that this is a bug. Sure, the README points out how to deal with Python3, but really: How hard would it be to fix this? You can’t expect python to point to a 2.x version anymore. Pretty much any new user who runs node install will run into this.

[cajlarsson]

+1
Ran into this today, PEP 394 defines this as a bug.

[rgbkrk]

👍

[TooTallNate]

Fixed by #527.