Foward NUMBER_OF_PROCESSORS environment variable by default

[AndreLouisCaron]

Hi there!

In one project, I have a Python 2.7-only dependency that uses multiprocessing.cpu_count() on Windows. This has a hidden dependency on the NUMBER_OF_PROCESSORS environment variable. This is quite confusing when porting this type of project into Tox.

Note: in Python 3.4+, this is API is superseeded by os.cpu_count() which does not depend on this environment variable.

The case is a bit weak (Python up to 3.3 only, plus multiprocessing is not popular on Windows), but I feel like the standard library should work by default inside a Tox test environment.

I'm willing to work on a patch if you would accept such a contribution!

[obestwalter]

Hi @AndreLouisCaron We have a growing list of special cases and for one think that your argument that tox should work out of the box with standard library is a compelling one, to add that also.