parsers: use PyVarObject_HEAD_INIT

The macro changed slightly in Python 3, introducing curly brackets
that somehow confuse Clang into issuing a ton of compiler warnings.
Using PyVarObject_HEAD_INIT makes these go away.

It's worth noting that the code is identical: the 2nd argument to
PyVarObject_HEAD_INIT is assigned to the ob_size field and is
inserted immediately after "PyObject_HEAD_INIT(type)" is generated.
Compilers are weird.

mercurial/parsers.c

@@ -2516,8 +2516,7 b' static PyGetSetDef index_getset[] = {'
 };
 
 static PyTypeObject indexType = {
-	PyObject_HEAD_INIT(NULL)
-	0,                         /* ob_size */
+	PyVarObject_HEAD_INIT(NULL, 0)
 	"parsers.index",           /* tp_name */
 	sizeof(indexObject),       /* tp_basicsize */
 	0,                         /* tp_itemsize */