Make sure that cached values from links_and_reverse_links_dict are not mutable inside (#826)

Yay python, it's great... In 20b45ba we
made sure that we'd return read only dicts from this function since it's
caching the result, but we kinda forgot that the value was a set and
thus was still mutable... This commits freezes said sets so now those
objects should be properly read only...

Author: Eijebong

src/taskgraph/graph.py

@@ -133,7 +133,11 @@ def links_and_reverse_links_dict(self):
         for left, right, _ in self.edges:
             forward[left].add(right)
             reverse[right].add(left)
-        return (ReadOnlyDict(forward), ReadOnlyDict(reverse))
+
+        return (
+            ReadOnlyDict({key: frozenset(value) for key, value in forward.items()}),
+            ReadOnlyDict({key: frozenset(value) for key, value in reverse.items()}),
+        )
 
     def links_dict(self):
         """