Migrate to pixi

[adityagoel4512]

This PR migrates this repository to use pixi
In the process:

• it updates the minimal python version to 3.9
• it adds tests on windows
• mypy runs properly - some typing inconsistencies emerged. I fixed a couple of trivial ones but have added # type: ignore for the others in case they require some more refactoring.
• the opsets are regenerated (with only some docstring changes that look valid). They were previously being generated from onnx 1.16 (because of the python 3.8 pin potentially?) when onnx 1.17 had already been released.
• license insertion

Checklist

• Added a CHANGELOG.rst entry

[cbourjau]

Thanks for this chore!