ENH: Expose type annotations declared in `_typing.py`
I often use pandas type annotations found in the `_typing` module: `FrameOrSeries`, `FrameOrSeriesUnion`, ...

It feels a bit weird to import stuff from a private module though. Would it make sense to expose *some* annotations in a `typing.py` module?

#### API breaking implications

I wouldn't expect any, `typing.py` could be an additional module that imports from `_typing.py`

#### Describe alternatives you've considered

Alternatively, annotations could be added to the main `__init__.py` so they can be imported with `pandas.*`.