Fixes Bug #0013433 - allow ancestor_too_old() tests to be done

Previous code did not clear the self.pset list between running descendant test and ancestor test, so  ancestor test code assumed all persons had already been checked.
This bug and fix is easier to evaluate if patch for bug #13431 has been applied - to improve debug output.

gramps/gen/utils/alive.py

@@ -615,6 +615,7 @@ def ancestors_too_old(person, year):
 
             return (None, None, "", None)
 
+        self.pset = set()
         try:
             # If there are ancestors that would be too old in the current year
             # then assume our person must be dead too.