README.md: update Python versions

Author: pseewald

README.md

@@ -25,7 +25,7 @@ fprettify is an auto-formatter for modern Fortran code that imposes strict white
 
 ## Requirements
 
-* Python 2.7 or Python 3.x
+* Python 3 (Python 2.7 no longer supported)
 * [ConfigArgParse](https://pypi.org/project/ConfigArgParse): optional, enables use of config file