Fix the Sphix dependency

Limit the version of Sphinx to 1.2.3 to resolve the
python 2.6 build failed issue.
The root cause of this is adding new typing package
dependency in latest Sphinx,and even the oldest typing
package can't support 2.6 build.

requirements/devel.txt

@@ -4,4 +4,4 @@ flake8<=3.0.3
 coverage
 argcomplete
 # docs
-Sphinx
+Sphinx<=1.2.3