Maintain compatibility with onnx 1.14

onnx 1.15 introduced some performance optimizations. We can forego
their usage if onnx<1.15 without loss of functionality.

src/spox/_attributes.py

@@ -210,9 +210,16 @@ def maybe(
         return cls(tuple(value), name) if value is not None else None
 
     def _to_onnx_deref(self) -> AttributeProto:
-        return make_attribute(
-            self._name, self.value, attr_type=self._attribute_proto_type
-        )
+        # the attr_type argument was only added in onnx>=1.15. It is a
+        # performance optimization for which we don't want to
+        # introduce a version bound. If we fail, we try again without
+        # it.
+        try:
+            return make_attribute(
+                self._name, self.value, attr_type=self._attribute_proto_type
+            )
+        except TypeError:
+            return make_attribute(self._name, self.value)
 
 
 class AttrFloat32s(_AttrIterable[float]):

tests/type_inference/test_scaler.py

@@ -16,4 +16,4 @@ def test_scaler_inference():
 def test_scaler_inference_fails_mismatched_lengths():
     (x,) = arguments(x=Tensor(np.float64, ("N", 3)))
     with pytest.raises(InferenceError):
-        op_ml.scaler(x, offset=[0.0, 0.1], scale=[1])
+        op_ml.scaler(x, offset=[0.0, 0.1], scale=[1.0])