heptapod-ci: use new v2.1 image This is finally catching up to ~3 years of tech debt.

heptapod-ci: move version prints closer to the start This makes debugging a lot easier if anything is to go wrong, and shows output earlier.

pytype: only try the hacky way of finding PYTHON if not provided This allows us to work in more environments, like when using pyenv. This syntax is compatible with all POSIX shells.

dummysmtpd: fix EOF handling on newer versions of OpenSSL Explanations inline.

test-install: add new glob for the upgrade notice in newer versions of pip We only globbed for the old warning, newer versions of pip use a cleaner one.

rust: use `.cargo/config.toml` instead of `.cargo/config` This has been deprecated for a while now and we don't support Rust versions that only understand the old path.

rust: apply clippy lints They are at most harmless and at best make the codebase more readable and simpler.

rust: change minimum supported version everywhere applicable This will help users and downstream packaging.

rustfmt: format the codebase with nightly-2024-07-16 The CI has moved to a newer nightly, which slightly changes how it wraps comments (which is the very option we use nightly for).

hghave: update detection of black version to a newer minimum The CI has moved to version 23.3.0, which is the last one to support 3.7 at runtime.