BUG: ensure to return writable buffer in __dataframe__ interchange for categorical column
- [x] closes https://github.com/pandas-dev/pandas/issues/48393
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature

LGTM. Not sure if the stalled CI without any test output is related to this PR or not. Thanks @jorisvandenbossche (CI stalling unrelated)