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.

Python/wordcode_helpers.h

@@ -39,6 +39,6 @@ write_op_arg(_Py_CODEUNIT *codestr, unsigned char opcode,
             *codestr++ = PACKOPARG(opcode, oparg & 0xff);
             break;
         default:
-            assert(0);
+            Py_UNREACHABLE();
     }
 }