rez.pip: Support python 2 executable on Windows (796)

[JeanChristopheMorinPerso]

Fixes #796.

On Windows, versions of Python less than 2 doesn't have versioned executable... So I'm basically just adding a check for windows and only append the version when not under windows.

[bfloch]

Another way to deal with it is https://www.python.org/dev/peps/pep-0397/

Python Launcher for Windows Version 3.7.5150.1013

usage:
py [launcher-args] [python-args] script [script-args]

Launcher arguments:

-2     : Launch the latest Python 2.x version
-3     : Launch the latest Python 3.x version
-X.Y   : Launch the specified Python version
     The above all default to 64 bit if a matching 64 bit python is present.
-X.Y-32: Launch the specified 32bit Python version
-X-32  : Launch the latest 32bit Python X version
-X.Y-64: Launch the specified 64bit Python version
-X-64  : Launch the latest 64bit Python X version
-0  --list       : List the available pythons
-0p --list-paths : List with paths

Not sure if it is guaranteed to be installed but once it is there is solves a bunch of issues.
It also respects #! shebang to some degree.

[JeanChristopheMorinPerso]

@bfloch thanks for pointing this out. I just don't know if it's available in all Python versions. I think we can go with the current solution as it's quite simple and won't bring us much support I think.

[instinct-vfx]

I think python launcher was added with 3.3 but i am not sure. 2.7 does not ship with it.

[instinct-vfx]

I think this is the easiest way to fix the issue.

[bfloch]

On windows both Python 2.7 and 3.x will be plain python.exe.
If you have and 3.4+ (I believe) installed you will have the launcher.

My thought was that a more reliable detection would be on windows to try py.exe and let it handle it. They already seem to have accounted for all the cases:
https://github.com/python/cpython/blob/master/PC/launcher.c

If it is not available just roll as is - since in this case it is likely you only have python 2.x installed anyway.

I just can't see how which + python.exe would have a chance to find the right version on Windows. It pretty much depends on the order of installation, no?

[nerdvegas]

Happy to go with this until proven otherwise that we need something more complex.

This and some other recent PR merges coming soon.

Cheers all
A

[bfloch]

Fair. I have not yet studied py.exe thoroughly to give a definite recommendation.