The issue #25150 modified pystate.h to hide _PyThreadState_Current. Sadly, this change broke the vmprof project:
https://mail.python.org/pipermail/python-dev/2016-January/142767.html

Attached patches adds a new private _PyThreadState_FastGet() function to get the current thread state but don't call Py_FatalError() if it is NULL.

The patch also uses replace direct access to _PyThreadState_Current with _PyThreadState_FastGet(), except inside ceval.c and pystate.c.

Calling Py_FatalError() to handle errors is not really a great API... Bad that's a different story, I don't want to break anything here.

I want to add the private function to Python 3.5.2 because I consider that the removal of the _PyThreadState_Current symbol is a regression introduced in Python 3.5.1.

We have no rule for the Python private API, it can change *anytime*.