parsers: fix memory leak in compute_phases_map_sets PySet_Add increments the reference of the added object to the set, see: https://hg.python.org/cpython/file/2.6/Objects/setobject.c#l379 Before this patch we were forgetting to decrement the reference count after adding objects to the phaseset. This patch fixes the issue and makes the reference count right so that these objects can be properly garbage collected.