fix: use correct type annotation for `padding_side`
Author: winstxnhdw
Pull request: yes

# What does this PR do?

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Fixes #36252


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Comments:
> Thanks! Can you also propagate it to `tokenization_utils.py` and `tokenization_utils_fast.py`
> 
> EDIT: Oops, seems we got another PR (#36326) which fixes the typing in all files. I'd opt for that one for being full and already ready to be merged

I prefer being more specific in my types since they show up in my intellisense but I am also aware that not everyone cares about this. Do let me know if you would still like a PR for this.