contrib: split Windows requirements into multiple files
Package support for Python 2 has diverged significantly. It is no
longer trivial to maintain a single requirements file that supports
both Python 2 and 3 because the set of packages and versions varies
wildly.
This commit split up the Windows requirements files so we have
variants for Python 2 and 3. As part of this, I also renamed the
files to have what I believe to be more reasonable naming ("win32"
felt like a weird identifier to me).
We can see that some package versions decreated on 2.7. This is
because the old pinned versions weren't compatible with Python 2.