bpo-42639: atexit now logs callbacks exceptions (GH-23771)

At Python exit, if a callback registered with atexit.register()
fails, its exception is now logged. Previously, only some exceptions
were logged, and the last exception was always silently ignored.

Add _PyAtExit_Call() function and remove
PyInterpreterState.atexit_func member. call_py_exitfuncs() now calls
directly _PyAtExit_Call().

The atexit module must now always be built as a built-in module.

Lib/test/test_threading.py

@@ -487,7 +487,6 @@ class ThreadTests(BaseTestCase):
                 if not pid:
                     print("child process ok", file=sys.stderr, flush=True)
                     # child process
-                    sys.exit()
                 else:
                     wait_process(pid, exitcode=0)