Add line to setup.cfg limiting to python >=3.5

This adds "Requires-Python: >=3.5" to the PKG-INFO file, which will then
in turn be processed by pypi and pip and prevent users on python2 from
being able to pip install zuul.

Change-Id: Ia2fe96bd151faf4f0c15be79872d4ddf9505b771

setup.cfg

@@ -6,6 +6,7 @@ description-file =
 author = Zuul Team
 author-email = zuul-discuss@lists.zuul-ci.org
 home-page = https://zuul-ci.org/
+python-requires = >=3.5
 classifier =
     Intended Audience :: Information Technology
     Intended Audience :: System Administrators