The attached script fails on master too. On bisecting could this be possibly caused due to 41e5ec377b (bpo-34769) ? Tagging Ned since it was introduced from 3.7.1rc2 and also backported to 3.6 .

➜  cpython git:(41e5ec377b) git checkout 41e5ec377b && make -s -j4 > /dev/null
HEAD is now at 41e5ec377b bpo-34769: Thread safety for _asyncgen_finalizer_hook(). (GH-9716)
➜  cpython git:(41e5ec377b) ./python.exe ../backups/bpo36403.py
F.
======================================================================
FAIL: test_main (__main__.TestAsyncIteratorFinally)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "../backups/bpo36403.py", line 30, in test_main
    self.assertTrue(it.finally_executed)
AssertionError: False is not true

----------------------------------------------------------------------
Ran 2 tests in 0.002s

FAILED (failures=1)
➜  cpython git:(41e5ec377b) git checkout 41e5ec377b~1 && make -s -j4 > /dev/null
Previous HEAD position was 41e5ec377b bpo-34769: Thread safety for _asyncgen_finalizer_hook(). (GH-9716)
HEAD is now at 0ce31d340b bpo-32962: Fix test_gdb failure in debug build with -mcet -fcf-protection -O0 (GH-9656)
➜  cpython git:(0ce31d340b) ./python.exe ../backups/bpo36403.py
..
----------------------------------------------------------------------
Ran 2 tests in 0.003s

OK