Updates readme to mention python 3.9 over 3.8

Author: neatc0der

README.md

@@ -14,7 +14,7 @@ This is a plugin and an extension for [mkdocs](https://github.com/mkdocs/mkdocs/
 
 This plugin was tested with, but is not limited to:
 
-* Python 3.8
+* Python 3.9
 * mkdocs 1.1
 
 ## Quickstart