Bump onnx from 1.8.0 to 1.9.0 in /ngraph/python

Bumps [onnx](https://github.com/onnx/onnx) from 1.8.0 to 1.9.0.
- [Release notes](https://github.com/onnx/onnx/releases)
- [Changelog](https://github.com/onnx/onnx/blob/master/docs/Changelog.md)
- [Commits](onnx/onnx@v1.8.0...v1.9.0)

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

ngraph/python/requirements_test.txt

@@ -2,7 +2,7 @@ flake8==3.8.4
 flake8-comprehensions==3.3.0
 flake8-docstrings==1.5.0
 flake8-quotes==3.2.0
-onnx==1.8.0
+onnx==1.9.0
 pydocstyle==5.1.1
 pytest==6.1.2
 retrying==1.3.3