bpo-30765: Avoid blocking when PyThread_acquire_lock() is asked not to (#2403)

bpo-30765: Avoid blocking when PyThread_acquire_lock() is asked not to (#2403)

* bpo-30765: Avoid blocking when PyThread_acquire_lock() is asked not to lock

This is especially important if PyThread_acquire_lock() is called reentrantly
(for example from a signal handler).

* Update 2017-06-26-14-29-50.bpo-30765.Q5iBmf.rst

* Avoid core logic when taking the mutex failed