chore: add Black code style badge

Author: nanotaboada

README.md

@@ -8,6 +8,7 @@
 [![codecov](https://codecov.io/gh/nanotaboada/python-samples-fastapi-restful/branch/master/graph/badge.svg?token=A1WNZPRQEJ)](https://codecov.io/gh/nanotaboada/python-samples-fastapi-restful)
 [![CodeFactor](https://www.codefactor.io/repository/github/nanotaboada/python-samples-fastapi-restful/badge)](https://www.codefactor.io/repository/github/nanotaboada/python-samples-fastapi-restful)
 [![codebeat badge](https://codebeat.co/badges/4c4f7c08-3b35-4b57-a875-bf2043efe515)](https://codebeat.co/projects/github-com-nanotaboada-python-samples-fastapi-restful-master)
+[![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
 
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