For dependencies, add links to PyPI pages

[Flimm]

This makes it more obvious which packages are Python packages, and how to install them with a tool like pip.

[pabs3]

@Flimm I note that the PyPI domain has changed, so the links in your PR are now pointing to an old domain that redirects to the new one. I suggest updating the links. Also one of the github.io links is broken and should point at the github.com repo instead.