style: fix mypy error

src/safeds/data/tabular/transformation/_one_hot_encoder.py

@@ -78,7 +78,7 @@ def __init__(
         self._new_column_names: list[str] | None = None
         self._mapping: dict[str, list[tuple[str, Any]]] | None = None  # Column name -> (new column name, value)[]
 
-    def __eq__(self, other):
+    def __eq__(self, other) -> bool:
         if not isinstance(other, OneHotEncoder):
             return NotImplemented
         elif self is other: