Allow passing python package URLs to the `bootstrap_python` function

[trungleduc]

Thank you very much for this amazing work.
For our use case, we would like to download Python packages from different servers. Would it be possible to make this function accept a dict of package names - package URLs and use it before trying to construct the URLs as of now?

[DerThorsten]

sure

[trungleduc]

Great! I can give it a shot