fix typing_extensions auto-import

[beauxq]

typeshed treats typing_extensions as stdlib
So when the auto-import inserts it, it inserts it in the stdlib section.
This fixes it to put it in the 3rd party section.

before:

from typing_extensions import override
from unittest import TestCase

after:

from unittest import TestCase

from typing_extensions import override

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[DetachHead]

i'm on the fence about this. on one hand typing_extensions can be considered first-party since it's an official package maintained in the @python org. but since it doesn't actually come with python i can also understand why you'd want it to be considered third-party like every other pypi package.

it looks like ruff's unsorted-imports rule also considers it third party, so i'm happy to do the same.