python: constraint onnxruntime version to fix uv/macos problems

`uv add onnxruntime` on MacOS currently fails as it resolves to 1.20.0,
even if that can't be used on MacOS. This, in turns, make `uv add
magika` fails as well.

Temporary fix is to help uv resolving to a proper version of
onnxruntime.

Close #801.