Bug#999850: More progress

[Jelmer Vernooij <jelmer@jelmer.uk>]

I've done another round of updates to the maturin package.

Mostly this has involved loosening dependencies so that it can use
versions of rust crates that are already in the archive. It's possible
that some of this will bite us further down the line (since some of
the dependencies are actually strict for a good reason), but
for the majority of crates this is fine. When that happens, we
can selectively bump the versions of those crate in the archive.

I've also dropped support for some optional features, including cross
building for Windows and Mac OS X, which requires crates that are
not packaged in Debian.

Right now, building is blocked on the following crates not being in
the archive:

 * cargo-options (packaged in debcargo-conf, will be uploaded shortly)
 * clap-complete-command >= 4 (not yet packaged)
  + this depends on a clap-complete-fig >= 4, newer than is currently in the
    archive
 * pyproject-toml
 * python-pkginfo
 * lddtree
 * pep440
 * minijinja2

Initially I'd probably disable optional features that depend on crates
that aren't in the archive. Some of these crates are:

 * multipart
 * native-tls-crate
 * keyring

If you're familiar with debcargo, then help packaging these crates
would be great towards getting maturin into Debian.

Cheers,

Jelmer