NETRC environment variable support

[atheriel]

The new repository auth feature supports reading credentials from .netrc files. It can sometimes be useful to point tools to a file other than ~/.netrc, and both pip and uv support a NETRC environment variable for doing this:

• For pip, which uses requests under the hood: https://github.com/psf/requests/blob/1764cc938efc3cc9720188dfa6c3852c45211aa0/src/requests/utils.py#L210
• For uv, which uses the netrc crate: https://github.com/gribouille/netrc/blob/f8b614440a8d45cae8a5bd90a43266d8f487b93b/src/lib.rs#L85-L105

Note that NETRC is not a universal standard (I believe it originates from the GNU project, though .netrc itself is much older), and some major tools don't support it, including:

• curl, which has no plans to; and
• git, which seems more open to it.

On balance, I think this still points towards our supporting it, though.