fix(dependency): add back typing-extensions for 3.9

[Lee-W]

Summary

Missing typing-extensions breaks "from libcst.codemod import CodemodContext"

This can be verified by

uv run --python 3.9 --with libcst==1.8.1 python -c "from libcst.codemod import CodemodContext"

it raises the following error

    from typing_extensions import ParamSpec
ModuleNotFoundError: No module named 'typing_extensions'

while it works fine for 3.10

uv run --python 3.10 --with libcst==1.8.1 python -c "from libcst.codemod import CodemodContext"

Test Plan

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[zsol]

Hmm looks like this was broken in 1.8.0 (specifically in #1294), but why didn't the tests catch it?

[zsol]

Ah I see, the test dependencies transitively include typing-extensions. Unfortunate.

[zsol]

Thanks for the fix!

[zsol]

I've put up #1359 to make sure this never happens again

[Lee-W]

Thanks for helping us merge it! It seems we excluded 1.8.0 last time (I wasn't aware), and this time 1.8.1 breaks my PR, which is why I find it. Glad to know 1.8.2 would work fine!