BUG: Categorical constructor not idempotent with ext dtype
- [x] closes #14190
- [ ] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

small comment but otherwise lgtm
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14191?src=pr) is 85.24% (diff: 100%)

> Merging [#14191](https://codecov.io/gh/pydata/pandas/pull/14191?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #14191   diff @@
==========================================
  Files           140        140          
  Lines         50555      50555          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43095      43094     -1   
- Misses         7460       7461     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [8af6264...4cad147](https://codecov.io/gh/pydata/pandas/compare/8af626474f6f314527a9ad3f15403aa2dd8c402d...4cad147d42826ca5c8c665a96a97ccd90d226726?src=pr)
 thanks!
