Lower Sphinx constraint

Sphinx 3.0.0 has introduced failures in several repos [0]. Until we can
identify the cause/fix for these failures, and given the point in the
release cycle, it might be best to keep this limited until we have some
more time to understand the issue.

[0] sphinx-doc/sphinx#7422

Change-Id: I8cf2a0932701b7023eef22b5cb2b6d3088891389
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

upper-constraints.txt

@@ -569,8 +569,8 @@ numpy===1.18.2;python_version=='3.6'
 numpy===1.18.2;python_version=='3.7'
 msgpack===0.6.2
 Sphinx===1.8.5;python_version=='2.7'
-Sphinx===3.0.0;python_version=='3.6'
-Sphinx===3.0.0;python_version=='3.7'
+Sphinx===2.4.4;python_version=='3.6'
+Sphinx===2.4.4;python_version=='3.7'
 oslo.config===7.0.0;python_version=='2.7'
 oslo.config===8.0.1;python_version=='3.6'
 oslo.config===8.0.1;python_version=='3.7'