Remove flake8-plugin-utils stubs (not before Jan 2024)

[Akuli]

A py.typed file was just added: afonasev/flake8-plugin-utils#78

[srittau]

There's been no release since. We can remove this six months after a release with py.typed.

[kkirsche]

For others coming to this in the future,

flake8-plugin-utils was last released on May 5, 2021 per https://pypi.org/project/flake8-plugin-utils/#history while the referenced pull request was submitted on December 6, 2021 and merged on January 13, 2022.

[sobolevn]

Released: afonasev/flake8-plugin-utils#78

[AlexWaygood]

A small bug in stubsabot was the reason why we didn't get an automated PR to mark these stubs as obsolete last night:

• Stubsabot: mark stubs as obsolete, even if the py.typed file was added in a micro version #10368

[AlexWaygood]

Marked as obsolete in #10373 🎉