Assertion Error in threading module when finalize_interpreter

Reproducible example code

Standalone setup is provided here : https://gist.github.com/abidrahmank/1ec549e13f600cd11eb70561c9223210

Just call make

Issue description

I have following workflow :

• I have a C library (crashloader.cpp) which will load other shared library plugins (threadcrash.cpp) and execute functions in that.

• Plugin libraries typically contains a function that creates an object (goo), execute it (execgoo) and delete it (delgoo).

• In my case, plugin embeds a Python script and execute it. So I initialize interpreter in object constructor and finalize it in destructor.

Issue : Everything works fine if I call exec in the same thread as creating the object. But the moment I call exec in a separate thread (which is my actual use case), I get following Assertion Error.

Exception ignored in: <module 'threading' from '......../lib/python3.6/threading.py'>
Traceback (most recent call last):
  File "......./lib/python3.6/threading.py", line 1299, in _shutdown
    assert tlock.locked()
AssertionError:  

Start with the standalone setup linked in the issue and run make to reproduce the assertion. Read crashloader.cpp and threadcrash.cpp, then trace the separate-thread execution through Python 3.6's threading.py _shutdown at line 1299. Done means the reproducible workflow no longer raises the assertion during interpreter finalization.