Make a few more safety improvements

[Qard]

This is mainly just about converting the RwLock to a Mutex to be absolutely sure there can never be one ensure_python_event_loop creating a new loop while another is about to write its own created loop. Rather than allowing and writes to interleave we just fully lock it down so any access has to wait until the full logic is complete. Because we never actually read this outside of construction, we don't really need to care about the perf difference between a Mutex and a RwLock.