Convert objectron proto.py back to using typing.List (#2559)

### What

The change from List -> list was introduced as part of:
 - #2361
 
However it seems to break objectron on python3.8:
 - #2557

Reverting just this bit causes it to work for me, though I don't fully
understand the intention or ramifications.

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested https://demo.rerun.io/pr/2559 (if applicable)

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PR Build Summary: https://build.rerun.io/pr/2559

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