Update python version in readme to 3.8 (#618)

Author: haifeng-jin

README.md

@@ -1,6 +1,6 @@
 # KerasNLP: Modular NLP Workflows for Keras
 [![](https://github.com/keras-team/keras-nlp/workflows/Tests/badge.svg?branch=master)](https://github.com/keras-team/keras-nlp/actions?query=workflow%3ATests+branch%3Amaster)
-![Python](https://img.shields.io/badge/python-v3.7.0+-success.svg)
+![Python](https://img.shields.io/badge/python-v3.8.0+-success.svg)
 ![Tensorflow](https://img.shields.io/badge/tensorflow-v2.5.0+-success.svg)
 [![contributions welcome](https://img.shields.io/badge/contributions-welcome-brightgreen.svg?style=flat)](https://github.com/keras-team/keras-nlp/issues)