Pin sphinx to <1.2

There is a bug in sphinx 1.2 that breaks doc builds.

See requirements commit 0e8862e958813a031fe7398440e8a3866b42a8e4
for details.

copied from https://review.openstack.org/#/c/61209/
Related-Bug: 1259511

Change-Id: Iea8ef70dd46faa1ad8f52c9b772dd7fa16902316