release 0.2.1

A few highlights:
- added initial `darwin` support
- added basic 1s throttling against `repology.org`
- improved error reporting when 4xx style errors (like 429) are served by HTTP

More detailed shortlog:

Artturin (5):
      src/flake.rs: Support darwin
      flake.lock: Update
      `cargo update`
      `cargo clippy --fix`
      Fix `unnecessary_find_map` `unnecessary_filter_map` lints
Sergei Trofimovich (9):
      Cargo.lock: run `cargo upgrade` and pull in all fresh depends
      refresh `flake.lock`
      README.md: document a workaround for "'/etc/nixos': ... is not owned by current user"
      src/repology.rs: make `version` field optional
      src/repology.rs: disable `--compressed` fetch in curl
      */*.rs: apply `rustfmt`
      src/repology.rs: run `curl` with `--no-progress-meter`
      src/repology.rs: use `cusl`s `--fail-with-body`
      src/repology.rs: implement trivial throttling bound by 1s

release 0.2.0