README: update supported python version

Author: zezha-msft

README.rst

@@ -148,7 +148,7 @@ Download a zip of the code via GitHub or PyPi. Then follow the same instructions
 Minimum Requirements
 --------------------
 
--  Python 2.7, 3.3, 3.4, 3.5, or 3.6.
+-  Python 2.7, 3.3-3.7.
 -  See setup.py for dependencies
 
 Usage