Make CollectionError pickleable

CollectionError could not be unpickled because the default
`Exception.__reduce__` implementation does not include any
attributes/values that weren't passed to the superclass constructor,
resulting in a failure when unpickling the exception in the parent
class because the `rule` argument wasn't given.

This updates the class to add a custom `__reduce__` that fixes the
ability to pickle/unpickle the class correctly, fixing the primary
issue with exceptions during linting resulting in broken process
pools.

Fix #381

ghstack-source-id: d8bd29c52d8320ee33a7f43937df6dc4e883d90d
Pull Request resolved: #401

src/fixit/config.py

@@ -69,6 +69,9 @@ def __init__(self, msg: str, rule: QualifiedRule):
         super().__init__(msg)
         self.rule = rule
 
+    def __reduce__(self):
+        return type(self), (*self.args, self.rule)
+
 
 def is_rule(obj: Type[T]) -> bool:
     """