TST: GH39443 - categorical datetime concat
- [x] closes https://github.com/pandas-dev/pandas/issues/39443
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry: Not added since I'm not sure whether it's necessary (there's an unanswered question in the issue regarding that) but am happy to do so provided some guidelines. Would it be simply modifying the latest file in `pandas/doc/source/whatsnew/`, i.e., `v1.3.0.rst`?
> plus would want input from others on if a whatsnew is necessary here

not needed @arredond can you merge master and ping on green @simonjayhawkins merged but no green. Apart from a couple of conda build fails I'm back to my original error:

```
>       tm.assert_equal(result, expected)
E       AssertionError: Attributes of DataFrame.iloc[:, 0] (column name="x") are different
E       
E       Attribute "dtype" are different
E       [left]:  CategoricalDtype(categories=['2021-01-01', '2021-01-02'], ordered=False)
E       [right]: datetime64[ns]
``` This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. Thanks for the PR, but appears this PR has gone stale and looks like it needs a little further investigation to solve the bug. Closing, but please let us know if you'd be interested in continuing and we'd be happy to reopen.