Fix memory leak in Python IDproperty iterator

Looping over IDProperty keys or calling iteritems leaked memory.

Error in original Python idproperty support from
8768707610

source/blender/python/generic/idprop_py_api.c

@@ -760,7 +760,6 @@ static PyObject *BPy_IDGroup_iter(BPy_IDProperty *self)
   iter->group = self;
   iter->mode = IDPROP_ITER_KEYS;
   iter->cur = self->prop->data.group.first;
-  Py_XINCREF(iter);
   return (PyObject *)iter;
 }
 
@@ -933,7 +932,6 @@ static PyObject *BPy_IDGroup_iter_items(BPy_IDProperty *self)
   iter->group = self;
   iter->mode = IDPROP_ITER_ITEMS;
   iter->cur = self->prop->data.group.first;
-  Py_XINCREF(iter);
   return (PyObject *)iter;
 }