Fix PyPI link/reference in readme

[PeterJCLaw]

No description provided.

[fralau]

Thanks a lot. Could you describe why the current pypi reference is not adequate and should be improved?

[PeterJCLaw]

On GitHub the current reference ends up as a clickable link (even if that wasn't your intention; I suspect it's a result of it being an image), however the link doesn't go anywhere especially useful. I had expected that when I clicked it I would end up at PyPI -- that's normally where the PyPI badge links to and helps confirm the link between that published package and this project.