Improve tuple.__add__ type

[JelleZijlstra]

Noticed this while browsing the stub. This feels safe but we'll see what mypy-primer has to say.

[github-actions]

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

kopf (https://github.com/nolar/kopf.git)
+ kopf/_cogs/structs/dicts.py:297: error: Unsupported operand types for + ("Tuple[str, ...]" and "Tuple[_K]")
+ kopf/_cogs/structs/dicts.py:323: error: Unsupported operand types for + ("Tuple[str, ...]" and "Tuple[_K]")
+ kopf/cli.py:95: error: Unsupported operand types for + ("Tuple[Union[str, Pattern[Any]], ...]" and "Tuple[Optional[str]]")

[Akuli]

The errors in kopf/.../dicts.py seem to be problems in the code being checked, as they expect an unbounded typevar to be str. But the second error doesn't make sense to me. It seems like Tuple[Optional[str]] somehow doesn't convert to Tuple[Optional[str], ...] so that it could be added to the other Tuple[Foo, ...].

[JelleZijlstra]

Yes, seems related to mypy treating fixed-size and arbitrary-size tuples as very different types internally.

[srittau]

This is an improvement. As the primer output shows it catches real type problems. The mypy bug it exposes seems to be fairly corner-casey to me and I think it's fine to # type: ignore it until it's fixed in mypy. Is there already a mypy bug about this?