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I'm spawning a dicussion buried in the way too long thread of https://bugs.python.org/issue6721 over here into its own specific issue to treat as a 3.7 release blocker for a rollback or repair decision before 3.7.4.
https://github.com/python/cpython/commit/3b699932e5ac3e76031bbb6d700fbea07492641d
I believe that was released in 3.7.1 is leading to a behavior regression for an application (the Fedora installer's libreswan kvmrunner?). Full details can be found in the messages of the other issue starting with:
 https://bugs.python.org/issue6721#msg329474
TL;DR - logging.Handler instances each have their own threading.Rlock. 
libreswan implements at least one logging.Handler subclass. That subclass's custom emit() implementation directly calls potentially many other sub-handlers emit() methods. Some of those emit() methods (such as logging.StreamHandler) call flush() which acquires the handler's lock.
So they've got a dependency between these two locks, the first's must be acquired before the second.
But the logging module APIs have no concept of sub-handlers and lock ordering.
I see many flaws with the libreswan code's design (I'm already ignoring the futility of threading + fork) but this still caused a behavior regression in the stable 3.7 release.
(more comments coming as followups to avoid a wall of text with too many topics)