fixup Py::cast_bound

[Icxolu]

I just noticed a that two small problems that slipped through #5289

• I forgot to revert the deprecations on Py::downcast_bound(_unchecked) (add Bound::cast and friends #5289 (comment))
• The new Py::cast_bound(_unchecked) methods should be on Py<T> and not Py<PyAny> (to be consistent with Bound)

Xref #5249

[davidhewitt]

Good catches, thanks 👍

[davidhewitt]

Needs #5362 to fix the merge failure, sorry.