#1733757: the interpreter would hang on shutdown, if the function set by sys.settrace

#1733757: the interpreter would hang on shutdown, if the function set by sys.settrace
calls threading.currentThread.

The correction somewhat improves the code, but it was close.
Many thanks to the "with" construct, which turns python code into C calls.

I wonder if it is not better to sys.settrace(None) just after
running the __main__ module and before finalization.