Ok, some comments about the patch (no "review" links appears so I'm gonna do it inline here):

- the doc addition needs a "versionadded" tag

- "The main thread is the thread that the OS creates to run application.": I would rephrase this "In normal conditions, the main thread is the thread from which the Python interpreter was started".

- in the tests:
+        self.assertEqual(data, "Thread-1\nTrue\nTrue\n")

Hmm, how do you know it will be called "Thread-1"?
I would give a specific name to the Thread, so as to make the test deterministic.

+        self.assertEqual(rc, 0)

You don't need this, it is already ensured by assert_python_ok().

- in threading.py, why doesn't _exitfunc() reuse the _main_thread global variable, instead of taking it as a parameter?