Allow non-uv based requirements.txt generation #565

Motivation

The CLI describes usethis tool requirements.txt thus:

Use a requirements.txt file exported from the uv lockfile.

It's worth thinking about how this can be less uv-centric. We could do unpleasant things like add pip as a dependency, subprocess pip and/or use the undocumented pip API. Another option is pip-tool but I suspect it would still be a matter of subprocessing:

https://pip-tools.readthedocs.io/en/latest/

Summary of desired enhancement
Decide and implement a way to generate requirements.txt incl. config without uv.