pathencode: use Py_SIZE directly

On Python 2, PyBytes_GET_SIZE is the same as PyString_GET_SIZE which
is the same as Py_SIZE which resolves to a struct member.

On Python 3, PyBytes_GET_SIZE is
"(assert(PyBytes_Check(op)),Py_SIZE(op))". The compiler barfs when
assigning to this version.

This patch simply changes PyBytes_GET_SIZE to Py_SIZE. On Python 2,
there is no effective change in behavior. On Python 3, we drop the
PyBytes_Check(). However, in all cases we have explicitly created
a PyBytesObject in the same function, so the PyBytes_Check() is
guaranteed to be true. Despite this, code changes over time, so
I've added added assert() in all callers so we can catch this in
debug builds.

With this patch, all mercurial.* C extensions now compile on Python 3
on my OS X machine. There are several compiler warnings and I'm sure
there are incompatibilities with Python 3, including possibly
segfaults. But it is a milestone.

mercurial/pathencode.c

@@ -171,7 +171,8 b' PyObject *encodedir(PyObject *self, PyOb'
 	newobj = PyBytes_FromStringAndSize(NULL, newlen);
 
 	if (newobj) {
-		PyBytes_GET_SIZE(newobj)--;
+		assert(PyBytes_Check(newobj));
+		Py_SIZE(newobj)--;
 		_encodedir(PyBytes_AS_STRING(newobj), newlen, path,
 			   len + 1);
 	}
@@ -638,7 +639,8 b' static PyObject *hashmangle(const char *'
 		memcopy(dest, &destlen, destsize, &src[lastdot],
 			len - lastdot - 1);
 
-	PyBytes_GET_SIZE(ret) = destlen;
+	PyBytes_Check(ret);
+	Py_SIZE(ret) = destlen;
 
 	return ret;
 }
@@ -750,7 +752,8 b' PyObject *pathencode(PyObject *self, PyO'
 		newobj = PyBytes_FromStringAndSize(NULL, newlen);
 
 		if (newobj) {
-			PyBytes_GET_SIZE(newobj)--;
+			PyBytes_Check(newobj);
+			Py_SIZE(newobj)--;
 			basicencode(PyBytes_AS_STRING(newobj), newlen, path,
 				    len + 1);
 		}