Update README.md (python versions)

Author: dmitry-lipetsk

README.md

@@ -1,5 +1,6 @@
 [![Build Status](https://app.travis-ci.com/postgrespro/testgres.pg_conf.svg?token=HgEMhyw9n7RCQkHCis4T&branch=master)](https://travis-ci.com/github/postgrespro/testgres.pg_conf)
 [![PyPI package version](https://badge.fury.io/py/testgres_postgres-configuration.svg)](https://badge.fury.io/py/testgres.postgres-configuration)
+[![PyPI python versions](https://img.shields.io/pypi/pyversions/testgres_postgres_configuration)](https://pypi.org/project/testgres.postgres-configuration)
 
 # PostgreSQL Configuration Python Library