chore: drop the unused typing-extensions lint dependency

[lesnik512]

The lint group's typing-extensions was inherited from modern-di core's template. Core added it to its own lint dependencies in its first commit, when its code imported typing_extensions. Nothing in this repo imports it, and ty does not report unresolved imports inside installed packages, so it was never needed. ty passes without it even against modern-di 3.5.0, whose code still had a TYPE_CHECKING import of typing_extensions.

Changes

• Drop typing-extensions from the lint dependency group.
• Drop it from the list of likely causes in .github/scripts/report-scheduled-failure.sh.

Verification

On 3.11: just lint-ci and just test-ci pass, with 100% coverage. Other dependencies still install typing-extensions here, so this run alone can't prove it unneeded. That proof comes from modern-di-flask and modern-di-pytest, where nothing else installs it, typing_extensions can't be imported, and ty and the tests still pass.

Release

None needed. Only the lint group and a CI script change, so nothing users install is affected.