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

Python/sysmodule.c

@@ -374,6 +374,30 @@ sys_exc_info_impl(PyObject *module)
 }
 
 
+/*[clinic input]
+sys.unraisablehook
+
+    unraisable: object
+    /
+
+Handle an unraisable exception.
+
+The unraisable argument has the following attributes:
+
+* exc_type: Exception type.
+* exc_value: Exception value.
+* exc_tb: Exception traceback, can be None.
+* obj: Object causing the exception, can be None.
+[clinic start generated code]*/
+
+static PyObject *
+sys_unraisablehook(PyObject *module, PyObject *unraisable)
+/*[clinic end generated code: output=bb92838b32abaa14 input=fdbdb47fdd0bee06]*/
+{
+    return _PyErr_WriteUnraisableDefaultHook(unraisable);
+}
+
+
 /*[clinic input]
 sys.exit
 
@@ -1672,6 +1696,7 @@ static PyMethodDef sys_methods[] = {
      METH_VARARGS | METH_KEYWORDS, set_asyncgen_hooks_doc},
     SYS_GET_ASYNCGEN_HOOKS_METHODDEF
     SYS_GETANDROIDAPILEVEL_METHODDEF
+    SYS_UNRAISABLEHOOK_METHODDEF
     {NULL,              NULL}           /* sentinel */
 };
 
@@ -2369,6 +2394,9 @@ _PySys_InitCore(_PyRuntimeState *runtime, PyInterpreterState *interp,
     SET_SYS_FROM_STRING_BORROW(
         "__breakpointhook__",
         PyDict_GetItemString(sysdict, "breakpointhook"));
+    SET_SYS_FROM_STRING_BORROW("__unraisablehook__",
+                               PyDict_GetItemString(sysdict, "unraisablehook"));
+
     SET_SYS_FROM_STRING("version",
                          PyUnicode_FromString(Py_GetVersion()));
     SET_SYS_FROM_STRING("hexversion",