Do not require explicit override for plugin-generated methods

[sterliakov]

Closes #17224.

Ensure that explicit-override error is never applied to functions which are not present in source code (generated by plugin), because there is no place to apply override.

[sterliakov]

I hope that tox internal failure is not caused by my interference...

[sterliakov]

It's transitive filelock dependency upgrade: current CI fails with 3.15.0, but succeeds with 3.14.0. I was able to reproduce this problem locally, pinning to filelock != 3.15.0 resolves the tox run problem. Would it be appropriate to add such a pin to this pull request (that breaks the "one change per PR" rule, but probably "internal dependency pin in CI" isn't that important)? Should I submit it separately (or probably investigate deeper)? Probably nothing, filelock maintainers are already engaged and work on this.

[sterliakov]

Cross-ref: tox-dev/filelock#337

[github-actions]

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