add 3.9

Author: jschlyter

README.md

@@ -1,7 +1,7 @@
 # cryptojwt
 
 ![License](https://img.shields.io/badge/license-Apache%202-blue.svg)
-![Python version](https://img.shields.io/badge/python-3.6%20%7C%203.7%203.8-blue.svg)
+![Python version](https://img.shields.io/badge/python-3.6%20%7C%203.7%7C%203.8%20%7C%203.9-blue.svg)
 
 An implementation of the JSON cryptographic specs JWS, JWE, JWK, and JWA [RFC 7515-7518] and JSON Web Token (JWT) [RFC 7519]