CLN: Simplify Block._astype

[TomAugspurger]

There's lots of nested logic there (plus 1 more level from #22343). Would be good to simplify this.

The components:

1. argument validation (could maybe do this elsewhere, not a big deal)
2. astyping to categorical is special cased. We'll want to remove that in favor of a generic astype system
3. Determine target_dtype -> Block type
4. _astype currently takes a values argument, which complicates things. Why is that? Why isn't it just self.values always?

[jorisvandenbossche]

Personally, I think we rather need to rethink the full astype-mechanism/dispatching first (what we were discussing in the PR #22343)

[jbrockmendel]

_astype currently takes a values argument, which complicates things. Why is that? Why isn't it just self.values always?

It is always None. It can't be removed until we get **kwargs out of NDFrame.astype 221b493#r34292758

[jbrockmendel]

I think this has been simplified as much as it can be (until deprecations are enforced in 2.0). Closing.