PyPI version tracking

For one of the following PRs, add the following bagde which will allow for tracking the PROMDENS version on PyPI.
`[![PyPI version](https://img.shields.io/pypi/v/promdens)](https://pypi.org/project/promdens/)` -> [![PyPI version](https://img.shields.io/pypi/v/promdens)](https://pypi.org/project/promdens/)
