Use | for Union types (and any other python 3.10+ features)

[TomNicholas]

Xarray is about to (pydata/xarray#8937), and we have no need to try to support older versions than xarray does (which supports python version for ~2.5 years after release IIRC).

This also allows us to do some nice things like use | instead of Union for union types.

[TomNicholas]

Oh huh we actually already support only 3.10+!

I'll rename this issue to instead be about using new features of 3.10+

[TomNicholas]

I actually can't find any examples of typing.Union in the codebase now, though I swear I saw some in some PRs. Closing.