docs: Update README with the new minimum Python version 3.6 (googleapis#26473)

Author: blakeli0

generator/README

@@ -34,7 +34,7 @@ Introduction
 
 Installation
 
-Python 2.7 is required.  Python3 is not supported.
+Python 3.6 is required.  Python 2 is not supported.
 
 Everything can be installed quickly with easy_install or pip. E.g.: