add required arg backtrack_causes to get_preferences() override

backtrack_causes is a required argument in get_preferences, as defined by the parent class resolvelib.AbstractProvider. Adding this argument allows the example to run without errors.

examples/reporter_demo.py

@@ -73,7 +73,7 @@ def __init__(self, index):
     def identify(self, requirement_or_candidate):
         return requirement_or_candidate.name
 
-    def get_preference(self, identifier, resolutions, candidates, information):
+    def get_preference(self, identifier, resolutions, candidates, information, backtrack_causes):
         return sum(1 for _ in candidates[identifier])
 
     def find_matches(self, identifier, requirements, incompatibilities):