bpo-1230540: Add threading.excepthook() (GH-13515)

Add a new threading.excepthook() function which handles uncaught
Thread.run() exception. It can be overridden to control how uncaught
exceptions are handled.

threading.ExceptHookArgs is not documented on purpose: it should not
be used directly.

* threading.excepthook() and threading.ExceptHookArgs.
* Add _PyErr_Display(): similar to PyErr_Display(), but accept a
  'file' parameter.
* Add _thread._excepthook(): C implementation of the exception hook
  calling _PyErr_Display().
* Add _thread._ExceptHookArgs: structseq type.
* Add threading._invoke_excepthook_wrapper() which handles the gory
  details to ensure that everything remains alive during Python
  shutdown.
* Add unit tests.

Include/internal/pycore_pylifecycle.h

@@ -107,6 +107,8 @@ PyAPI_FUNC(int) _Py_HandleSystemExit(int *exitcode_p);
 PyAPI_FUNC(PyObject*) _PyErr_WriteUnraisableDefaultHook(PyObject *unraisable);
 
 PyAPI_FUNC(void) _PyErr_Print(PyThreadState *tstate);
+PyAPI_FUNC(void) _PyErr_Display(PyObject *file, PyObject *exception,
+                                PyObject *value, PyObject *tb);
 
 #ifdef __cplusplus
 }