CI: run tests and lints on every PR #7

CI only builds wheels at the moment. Nothing runs the tests.

• Add pytest and maturin as dev dependencies (.venv doesn't even have pytest right now)
• New test job on Linux, macOS and Windows, with the oldest and newest Python we support: cargo fmt --check, cargo clippy --all-targets -- -D warnings, cargo test, maturin develop, pytest
• Clippy has about 59 existing warnings. Clean those up in a separate PR first (or allow them with a comment saying why), so this PR stays small
• Also check every commit in the PR, not only the last one (for example git rebase --exec '<checks>' origin/dev-v2 in a job). Since PRs are rebase-merged, a broken commit in the middle would land on dev-v2
• Make the job a required check on dev-v2

Done when: a PR with a failing test or a clippy warning in any of its commits can't be merged.

Depends on: #4, #5. Plan: WP-1.

Part of #3.