Fix singledispatch register signature

[flying-sheep]

Fixes #13560

Available since Python 3.11 according to the docs.

It also supports _UnionGenericAlias values (i.e. values resulting from typing Union[X, Y]), but idk if it’s possible to express that.

[github-actions]

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

ibis (https://github.com/ibis-project/ibis)
- ibis/backends/polars/compiler.py:745: note:     def register(self, cls: type[Any], func: None = ...) -> Callable[[Callable[..., Any]], Callable[..., Any]]
+ ibis/backends/polars/compiler.py:745: note:     def register(self, cls: type[Any] | UnionType, func: None = ...) -> Callable[[Callable[..., Any]], Callable[..., Any]]
- ibis/backends/polars/compiler.py:745: note:     def register(self, cls: type[Any], func: Callable[..., Any]) -> Callable[..., Any]
+ ibis/backends/polars/compiler.py:745: note:     def register(self, cls: type[Any] | UnionType, func: Callable[..., Any]) -> Callable[..., Any]

[srittau]

Thanks!