Summary:
Python 3 seems to have fixed the long standing codepage issue with
python on Windows. To be able to reliably build the documentation
we should use the Python 3 version of Sphinx.

See `https://docs.python.org/dev/whatsnew/3.3.html#codecs` and
`https://bugs.python.org/issue13216`. Sphinx also supports Python 3.

Oddly enough `python3-sphinx` is registered as `sphinx-build.exe`
instead of `sphinx-build3.exe`, so the changes in `configure.ac` can
be reverted.

install with `pacman -S mingw-w64-$(uname -m)-python3-sphinx`

Test Plan: ./validate

Reviewers: austin, thomie, bgamari

Reviewed By: bgamari

Subscribers: erikd

Differential Revision: https://phabricator.haskell.org/D1417

GHC Trac Issues: #11021