add supported py versions to readme.md

README.md

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+[![Supported versions](https://img.shields.io/pypi/pyversions/hydrolib-core.svg)](https://pypi.org/project/hydrolib-core)
 
 # HYDROLIB-core
 HYDROLIB-core is the core library of Python wrappers around the D-HYDRO model files (input and output) and model engines (kernel libraries).