Bump onnx from 1.18.0 to 1.21.0 in /src/bindings/python

Bumps [onnx](https://github.com/onnx/onnx) from 1.18.0 to 1.21.0.
- [Release notes](https://github.com/onnx/onnx/releases)
- [Changelog](https://github.com/onnx/onnx/blob/main/docs/Changelog-ml.md)
- [Commits](onnx/onnx@v1.18.0...v1.21.0)

---
updated-dependencies:
- dependency-name: onnx
  dependency-version: 1.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

src/bindings/python/constraints.txt

@@ -22,4 +22,4 @@ docopt~=0.6.2
 paddlepaddle==3.0.0
 tensorflow>=1.15.5,<2.21.0
 protobuf>=3.21.12,<7.0.0
-onnx==1.18.0
+onnx==1.21.0