Fixed type hint

[radarhere]

An error has started occurring in main - https://github.com/python-pillow/Pillow/actions/runs/11677258842/job/32514847792

setup.py:1010: error: Argument 2 to "Extension" has incompatible type
"list[str]"; expected "list[Union[str, PathLike[str]]]"  [arg-type]
        Extension("PIL._imaging", files),
                                  ^~~~~
setup.py:1010: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
setup.py:1010: note: Consider using "Sequence" instead, which is covariant
Found 1 error in 1 file (checked 294 source files)