Fix iteration over union (when self type is involved)

[hauntsaninja]

Fixes #17945

[github-actions]

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

streamlit (https://github.com/streamlit/streamlit)
- lib/tests/streamlit/elements/lib/column_config_utils_test.py:385:30: error: Invalid index type "str" for "Union[ColumnConfig, str, None]"; expected type "Union[SupportsIndex, slice]"  [index]
+ lib/tests/streamlit/elements/lib/column_config_utils_test.py:385:30: error: Invalid index type "str" for "str"; expected type "Union[SupportsIndex, slice]"  [index]

spack (https://github.com/spack/spack)
- lib/spack/spack/util/spack_yaml.py:490: error: Item "dict_items[Any, Any]" of "Iterator[tuple[Any, Any]] | dict_items[Any, Any] | enumerate[Any]" has no attribute "__next__"  [union-attr]

[JukkaL]

Thanks for the fix!