COMPAT: make Categorical._codes settable again
This is a follow-up on https://github.com/pandas-dev/pandas/pull/40033, which changes the `_code` attribute of a Categorical into a property without a setter.

This broke dask / fastparquet, so this PR is making it settable again. We could deprecate the setter, though, at the same time.
> We could deprecate the setter, though, at the same time

+1 > This broke dask / fastparquet, so this PR is making it settable again. We could deprecate the setter, though, at the same time.

new PR? or this one? Let's merge this to fix dask's CI. We can deprecate it in a follow-up / together with providing an alternative. I opened https://github.com/pandas-dev/pandas/issues/40606 marked for 1.3, so we don't forget this