Fix type narrowing for types.EllipsisType

Fixes python#17002

mypy/types.py

@@ -1524,7 +1524,7 @@ def is_singleton_type(self) -> bool:
         return (
             self.type.is_enum
             and len(self.get_enum_values()) == 1
-            or self.type.fullname == "builtins.ellipsis"
+            or self.type.fullname in {"builtins.ellipsis", "types.EllipsisType"}
         )
 
     def get_enum_values(self) -> list[str]: