feat(pypi): allow configuration of upstream URL #292

Reviewing #290 I've noticed that pypi.org is currently the only possible upstream. Python had a lot of adoption in the ML domain where packages are huge but also built for specific architectures and platforms. pytorch is a prominent example which also provides it's own index.

If we could configure the pypi upstream like for npm, Debian or others, we could cover those use cases too by simply deploying another instance with the correct configuration.