bpo-36829: Add sys.unraisablehook() (GH-13187)

Add new sys.unraisablehook() function which can be overridden to
control how "unraisable exceptions" are handled. It is called when an
exception has occurred but there is no way for Python to handle it.
For example, when a destructor raises an exception or during garbage
collection (gc.collect()).

Changes:

* Add an internal UnraisableHookArgs type used to pass arguments to
  sys.unraisablehook.
* Add _PyErr_WriteUnraisableDefaultHook().
* The default hook now ignores exception on writing the traceback.
* test_sys now uses unittest.main() to automatically discover tests:
  remove test_main().
* Add _PyErr_Init().
* Fix PyErr_WriteUnraisable(): hold a strong reference to sys.stderr
  while using it

Doc/c-api/exceptions.rst

@@ -72,6 +72,9 @@ Printing and clearing
 
 .. c:function:: void PyErr_WriteUnraisable(PyObject *obj)
 
+   Call :func:`sys.unraisablehook` using the current exception and *obj*
+   argument.
+
    This utility function prints a warning message to ``sys.stderr`` when an
    exception has been set but it is impossible for the interpreter to actually
    raise the exception.  It is used, for example, when an exception occurs in an
@@ -81,6 +84,8 @@ Printing and clearing
    in which the unraisable exception occurred. If possible,
    the repr of *obj* will be printed in the warning message.
 
+   An exception must be set when calling this function.
+
 
 Raising exceptions
 ==================