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

Even with https://github.com/channable/opsqueue/issues/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 (https://github.com/PyO3/pyo3/issues/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.