perf: disallow __dunder__ attributes on Deferred (#10888)

Author: NickCrews

ibis/common/deferred.py

@@ -94,6 +94,8 @@ def __repr__(self):
         return repr(self._resolver) if self._repr is None else self._repr
 
     def __getattr__(self, name):
+        if name.startswith("__") and name.endswith("__"):
+            raise AttributeError(name)
         return Deferred(Attr(self, name))
 
     def __iter__(self):