check for py 3.5 minimum requirement in setup.py

we do not test on 3.4 any more and have dropped support for it.
so we better enforce it in setup.py also.

setup.py

@@ -12,7 +12,7 @@ from distutils.core import Command
 
 import textwrap
 
-min_python = (3, 4)
+min_python = (3, 5)
 my_python = sys.version_info
 
 if my_python < min_python: