Reintroduce Python 3.9 support for ModernBERT

[tomaarsen]

What does this PR do?

Fixes https://huggingface.co/answerdotai/ModernBERT-base/discussions/21

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Reproduction

from typing import Tuple

foo: Tuple[int] | Tuple[str]

This script fails in Python 3.9:

Traceback (most recent call last):
  File "[sic]/demo_py39_test.py", line 4, in <module>
    foo: Tuple[int] | Tuple[str]
TypeError: unsupported operand type(s) for |: '_GenericAlias' and '_GenericAlias'

But works in Python 3.10+. See also https://bugs.python.org/issue42233

After this PR

With this PR in place, ModernBERT can be ran in Python 3.9.

Who can review?

@ArthurZucker
cc @warner-benjamin @KoichiYasuoka

• Tom Aarsen

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[KoichiYasuoka]

Thank you for catching this @tomaarsen

[warner-benjamin]

LGTM

[LysandreJik]

Thank you Tom!