Some final touches for variadic types support

[ilevkivskyi]

I decided to go again over various parts of variadic types implementation to double-check nothing is missing, checked interaction with various "advanced" features (dataclasses, protocols, self-types, match statement, etc.), added some more tests (including incremental), and greped for potentially unhandled cases (and did found few crashes). This mostly touches only variadic types but one thing goes beyond, the fix for self-types upper bound, I think it is correct and should be safe.

If there are no objections, next PR will flip the switch.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

openlibrary (https://github.com/internetarchive/openlibrary)
+ openlibrary/plugins/openlibrary/lists.py:272: error: Unused "type: ignore" comment  [unused-ignore]
+ openlibrary/plugins/openlibrary/lists.py:317: error: Unused "type: ignore" comment  [unused-ignore]

prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/events/worker.py:48: error: Signature of "instance" incompatible with supertype "QueueService"  [override]
- src/prefect/events/worker.py:48: note:      Superclass:
- src/prefect/events/worker.py:48: note:          @classmethod
- src/prefect/events/worker.py:48: note:          def instance(cls, *args: Any) -> EventsWorker
- src/prefect/events/worker.py:48: note:      Subclass:
- src/prefect/events/worker.py:48: note:          @classmethod
- src/prefect/events/worker.py:48: note:          def instance(cls, client_type: type[EventsClient] | None = ...) -> EventsWorker
- src/prefect/logging/handlers.py:70: error: Signature of "instance" incompatible with supertype "QueueService"  [override]
- src/prefect/logging/handlers.py:70: note:      Superclass:
- src/prefect/logging/handlers.py:70: note:          @classmethod
- src/prefect/logging/handlers.py:70: note:          def instance(cls, *args: Any) -> APILogWorker
- src/prefect/logging/handlers.py:70: note:      Subclass:
- src/prefect/logging/handlers.py:70: note:          @classmethod
- src/prefect/logging/handlers.py:70: note:          def instance(cls) -> APILogWorker

mongo-python-driver (https://github.com/mongodb/mongo-python-driver)
+ bson/son.py:71: error: Unused "type: ignore" comment  [unused-ignore]

[hauntsaninja]

Ah, just saw I didn't do merge right, thanks for fixing! Tests and changes I understand look good

[ilevkivskyi]

@jhance @JukkaL To speed things up I am going to merge this later today, if there are no more comments.

[JukkaL]

Did a quick review pass, looks good.