fix: python version compatibility and mypy issues

[begumcig]

Description

transformers == 4.51 introduced some mypy errors as referenced huggingface/transformers#37339
Also addresses compatibility issues with Python <3.10 caused by the use of the | (union) operator.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[llcnt]

I re-ran some tests (no time to run them all unfortunately) just to be sure, and looks good :)