backport venv module

[graingert]

https://github.com/python/cpython/tree/3.7/Lib/venv/

[arizvisa]

As a result of PR #117, the venv module will likely need to be rewritten a bit for the Windows platform as the PYTHONHOME used to locate all of the available modules is determined by the location of the Python DLL, so you can literally just copy the directory around and run python from it without actually needing to "activate" it in your shell.

Installation of packages should depend on site.getsitepackages() which uses sys.prefix and sys.execprefix, so just make sure you run the correct executable. Posix-compliant platforms also should still work the same which means you can likely just use the module as-is and it will work as you intend.

I can look at making the contents of that dir into a PR once my latest PR gets merged, but in the meantime you should be fine.