Update badge to Python 3.9

Author: bact

README.md

@@ -3,7 +3,7 @@
   <h1>PyThaiNLP: Thai Natural Language Processing in Python</h1>
   <a href="https://www.repostatus.org/#active"><img alt="Project Status: Active – The project has reached a stable, usable state and is being actively developed." src="https://www.repostatus.org/badges/latest/active.svg"/></a>
   <a href="https://pypi.python.org/pypi/pythainlp"><img alt="pypi" src="https://img.shields.io/pypi/v/pythainlp.svg"/></a>
-  <a href="https://www.python.org/downloads/release/python-370/"><img alt="Python 3.8" src="https://img.shields.io/badge/python-3.8-blue.svg"/></a>
+  <a href="https://www.python.org/downloads/"><img alt="Python 3.9" src="https://img.shields.io/badge/python-3.9-blue.svg"/></a>
   <a href="https://opensource.org/licenses/Apache-2.0"><img alt="License" src="https://img.shields.io/badge/License-Apache%202.0-blue.svg"/></a>
   <a href="https://www.codacy.com/gh/PyThaiNLP/pythainlp/dashboard?utm_source=github.com&amp;utm_medium=referral&amp;utm_content=PyThaiNLP/pythainlp&amp;utm_campaign=Badge_Grade"><img src="https://app.codacy.com/project/badge/Grade/5821a0de122041c79999bbb280230ffb"/></a>
   <a href="https://coveralls.io/github/PyThaiNLP/pythainlp?branch=dev"><img alt="Coverage Status" src="https://coveralls.io/repos/github/PyThaiNLP/pythainlp/badge.svg?branch=dev"/></a>