bpo-31338 (#3374)

* Add Py_UNREACHABLE() as an alias to abort().
* Use Py_UNREACHABLE() instead of assert(0)
* Convert more unreachable code to use Py_UNREACHABLE()
* Document Py_UNREACHABLE() and a few other macros.

Objects/abstract.c

@@ -1984,7 +1984,7 @@ _PySequence_IterSearch(PyObject *seq, PyObject *obj, int operation)
                 goto Done;
 
             default:
-                assert(!"unknown operation");
+                Py_UNREACHABLE();
             }
         }