add RwLockExt trait (#5435)

* add RwLockExt trait to detach from Python while blocking on a RwLock

* implement RwLockExt for lock_api::RwLock and Arc<RwLock>

* add test of parking_lot::RwLock (via lock_api)

* fix typo in test name

* fix docs

* check reader blocks writer case / more checks for poisoned rwlock

Author: tdyas

newsfragments/5435.added.md

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+Added the `pyo3::sync::RwLockExt` trait to allow detaching from the Python interpreter while blocking to acquire a rwlock. The trait is implemented for `std::sync::RwLock` as well as `parking_lot`'s / `lock_api`'s `RwLock`.