cext: fix Python 3.11 compatibility - Py_SIZE is not an lvalue (issue6610)

Py_SIZE was made a static inline function during Python 3.10 development, as
described on https://vstinner.github.io/c-api-opaque-structures.html .
e92ca942ddca updated the Mercurial code base accordingly, but somehow missed a
couple of cases introduced long time ago in a8c948ee3668.

The Python change was dropped for 3.10, but is coming back again in 3.11 .

mercurial/cext/pathencode.c

@@ -176,7 +176,7 b' PyObject *encodedir(PyObject *self, PyOb'
 
 	if (newobj) {
 		assert(PyBytes_Check(newobj));
-		Py_SIZE(newobj)--;
+		Py_SET_SIZE(newobj, Py_SIZE(newobj) - 1);
 		_encodedir(PyBytes_AS_STRING(newobj), newlen, path, len + 1);
 	}
 
@@ -791,7 +791,7 b' PyObject *pathencode(PyObject *self, PyO'
 
 		if (newobj) {
 			assert(PyBytes_Check(newobj));
-			Py_SIZE(newobj)--;
+			Py_SET_SIZE(newobj, Py_SIZE(newobj) - 1);
 			basicencode(PyBytes_AS_STRING(newobj), newlen, path,
 			            len + 1);
 		}