Allow doc building using earlier versions of Sphinx.

Review Request #10779 — Created Nov. 3, 2019 and discarded — Latest diff uploaded


Django Evolution


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