Make .str/.dt available for Series of type category with string/datetime
If a series is a type category and the underlying Categorical has categories of type string or datetime, then make it possible to use the `.str`/`.dt` assessor on such a series.

The string/dt methods work on the categories (and therefore fast if we have
only a few categories), but return a Series with a dtype other than
category (integer, boolean, string,...), so that it is no different if we use
`.str` / `.dt` on a series of type string or of type category.

The main reason for that is that I think things like `s.str.slice(...) + s.str.slice(...)` should work.

Closes: https://github.com/pydata/pandas/issues/10661
