Feature: Allow upip to install from test.pypi.org

[brainelectronics]

Relates a little bit to #46

For initial testing it would be good to enable a package installation from test.pypi.org to avoid unnecessary version dumps every time.

For sure you can simply use the non-micropython pip and install it to the unix port of micropython for testing purposes.

I just want go gather some ideas and opinions before creating a pull request for a feature nobody needs. Overall I'm thinking of an additional argument at the install function to allow another installation root URL. This might be also good for self hosted packages or other package hosts, gitlab, artifact repository ...