Add typing stubs (`.pyi` files) for PyO3-generated Python definitions

[isomorpheme]

Even with #8 resolved, type checkers will treat types that are exclusively defined via C bindings as Any. Typing stubs would fix this, but PyO3 currently does not generate them (PyO3/pyo3#5137). Manually maintaining them would be annoying because it'd have to be kept in sync with the Rust definitions of the types, but it could still be valuable as a temporary solution.

[isomorpheme]

It might be possible to already let PyO3 do this, Maturin recently added a flag to enable it: https://github.com/PyO3/maturin/releases/tag/v1.13.0

So we'd just have to upgrade Maturin (we're on 1.10.0 at the moment). The only thing is that PyO3 might not support every kind of annotation we need yet.