[3.6] bpo-30765: Avoid blocking when PyThread_acquire_lock() is asked not to (GH...

[3.6] bpo-30765: Avoid blocking when PyThread_acquire_lock() is asked not to (GH-2403) (#2418)

* 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
(cherry picked from commit f84ac420c2af98339678744953869cad3c253281)