python: use Py_TYPE instead of looking directly into PyObject_HEAD

Py_TYPE works on both Python2 and Python3, while internals of
PyObject_HEAD have changed.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

tools/python/xen/lowlevel/xc/xc.c

@@ -2670,7 +2670,7 @@ static void PyXc_dealloc(XcObject *self)
         self->xc_handle = NULL;
     }
 
-    self->ob_type->tp_free((PyObject *)self);
+    Py_TYPE(self)->tp_free((PyObject *)self);
 }
 
 static PyTypeObject PyXcType = {

tools/python/xen/lowlevel/xs/xs.c

@@ -922,7 +922,7 @@ static void xshandle_dealloc(XsHandle *self)
 
     Py_XDECREF(self->watches);
 
-    self->ob_type->tp_free((PyObject *)self);
+    Py_TYPE(self)->tp_free((PyObject *)self);
 }
 
 static PyTypeObject xshandle_type = {