Bump sphinx version: fixes (again) PDF for french.

See: python/python-docs-fr#199
and: sphinx-doc/sphinx#5130

Author: JulienPalard

requirements.txt

@@ -16,6 +16,6 @@ pytz==2018.4
 requests==2.19.1
 six==1.11.0
 snowballstemmer==1.2.1
-Sphinx==1.7.5
+Sphinx==1.7.6
 sphinxcontrib-websupport==1.1.0
 urllib3==1.23