PTFM-17722 Windows support for 'make python'
PTFM-17722 Windows support for 'make python'

Summary:
Initial support for running the 'make python' target on Windows.

Incorporates Arina's fix for locating the virtualenv activate.bat script on Windows (thanks!), as well as special steps for handling the temporary wheels dir on Windows while preparing to create a package dir for the dx-toolkit archive.

Test Plan: Built dx.exe using 'make python' on Windows and performed manual tests with the resulting executable.

Reviewers: nicolasbockg

Reviewed By: nicolasbockg

Subscribers: psung, arina

Differential Revision: https://dnanexus.phacility.com/D919

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