Python cryptography, Rust, and Gentoo

Forced to use the latest features? Not likely. Allowed them to remove bad patterns, simplify others, or just use new ones? Sure. There's the nightly-only `-Z minimal-versions` flag to use the *minimum* declared versions of dependencies, but discussions about stabilizing it and making it work across the ecosystem haven't gotten very far (and I've submitted over a dozen PRs to make my dependencies work with the flag).