Allow doc building using earlier versions of Sphinx.
Allow doc building using earlier versions of Sphinx.

A recent change required Sphinx 2.1.1+ for doc building, but this
version isn't available on Python 2.7. We want to support building using
both Python 2.x and 3.x, particularly within CI environments. This
change relaxes the version restriction to the latest 1.x release
(1.8.5).

beanbaginc/django-evolution