Allow upip to install from custom URLs (not just pypi.org)

[redstreet]

It would be great if upip supported private repos outside pypi.org. See private-pypi.

It looks like the code change is simple, to parameterize the website via an api like upip.set_pypi_url(), which will be used below:

https://github.com/micropython/micropython-lib/blob/eae01bd4e4cd1b22d9ccfedbd6bf9d879f64d9bd/upip/upip.py#L159

Would you entertain a PR for this if I submitted one? Thanks!