Remove classifier for Python 3.4 for end-of-life. (#7535)

* Remove classifier for Python 3.4 for end-of-life.

* Update supported versions in Client README, Contributing Guide

Author: crwilcox

README.rst

@@ -16,7 +16,7 @@ common helpers used by all Google API clients. For more information, see the
 
 Supported Python Versions
 -------------------------
-Python >= 3.4
+Python >= 3.5
 
 Deprecated Python Versions
 --------------------------