Summary:
Fix building new Sphinx documenation on Windows in msys2 using Awson's patch on #11021.

Install Sphinx using `pacman -S mingw-w64-$(uname -m)-python2-sphinx`

Test Plan: Apply patch and ./validate

Reviewers: thomie, bgamari, austin

Reviewed By: thomie, bgamari

Subscribers: erikd

Differential Revision: https://phabricator.haskell.org/D1408

GHC Trac Issues: #11021