Fix union callees with functools.partial

[JukkaL]

Fixes #17741.

[github-actions]

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

setuptools (https://github.com/pypa/setuptools)
- setuptools/config/setupcfg.py:303: error: "Callable[[str, Any], None] | Any" not callable  [misc]

pip (https://github.com/pypa/pip)
- src/pip/_internal/utils/misc.py:132: error: "Callable[[FunctionType, Path, BaseException], Any] | Callable[[FunctionType, Path, tuple[type[BaseException], BaseException, TracebackType]], Any]" not callable  [misc]
+ src/pip/_internal/utils/misc.py:132: error: Unexpected keyword argument "onexc"  [call-arg]

[hauntsaninja]

Thanks! Simpler than the changes I thought about when I saw the issue. mypy_primer checks out too