fix list.__add__ to use the expected type

[KotlinIsland]

ergh, stupid github ui

[github-actions]

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

zulip (https://github.com/zulip/zulip)
+ zerver/lib/onboarding_steps.py:65: error: Unsupported operand types for + ("list[OneTimeNotice]" and "list[OneTimeAction]")  [operator]

prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/utilities/callables.py:579: error: Incompatible types in assignment (expression has type "list[None]", variable has type "list[Optional[expr]]")  [assignment]
- src/prefect/utilities/callables.py:579: note: "list" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
- src/prefect/utilities/callables.py:579: note: Consider using "Sequence" instead, which is covariant
- src/prefect/utilities/callables.py:581: error: Unsupported operand types for + ("list[None]" and "list[Optional[expr]]")  [operator]

pydantic (https://github.com/pydantic/pydantic)
- pydantic/aliases.py:29: error: Incompatible types in assignment (expression has type "list[str]", variable has type "list[int | str]")  [assignment]
- pydantic/aliases.py:29: note: "list" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
- pydantic/aliases.py:29: note: Consider using "Sequence" instead, which is covariant
- pydantic/aliases.py:29: error: Argument 1 to "list" has incompatible type "tuple[str | int, ...]"; expected "Iterable[str]"  [arg-type]

meson (https://github.com/mesonbuild/meson)
+ mesonbuild/modules/i18n.py:165:43: error: Unsupported operand types for + ("list[File]" and "list[CustomTarget]")  [operator]