docs(readme): add Python 3.9 to badges (#584)

* Add Python 3.9 to badges

* Change Python version to >= 3.6

README.md

@@ -4,7 +4,7 @@
 
 [![license](https://img.shields.io/badge/license-MIT-blue.svg)](/LICENSE)
 [![pypi version](https://badge.fury.io/py/diagrams.svg)](https://badge.fury.io/py/diagrams)
-![python version](https://img.shields.io/badge/python-3.6%2C3.7%2C3.8-blue?logo=python)
+![python version](https://img.shields.io/badge/python-%3E%3D%203.6-blue?logo=python)
 ![Run tests](https://github.com/mingrammer/diagrams/workflows/Run%20tests/badge.svg?branch=master)
 [![todos](https://badgen.net/https/api.tickgit.com/badgen/github.com/mingrammer/diagrams?label=todos)](https://www.tickgit.com/browse?repo=github.com/mingrammer/diagrams)
 ![contributors](https://img.shields.io/github/contributors/mingrammer/diagrams)