Some users are installing Black as a dependency in their project. Having
a _version.py in site-packages is asking for a conflict sooner or later.
Ideally we shouldn't require a separate version file at all, that's an
additional import we need to make. But I'll leave that bikeshedding for
a different time.
* Fix multiprocessing support for Windows binary
The black and blackd binaries generated for Windows builds would fail on
reformatting multiple files due to a Windows-specific
multiprocessing issue. Fix by calling freeze_support() as
described in Python docs.
