Fix ns precision isoformat
- [x] closes #53020
- [x] [Tests added and passed] Added new tests
- [x] All [code checks passed]
- [x] Added [type annotations]
- [x] Added an entry in the latest `doc/source/whatsnew/v2.1.0.rst` file

Please ensure you run precommit to fix code check issues: https://pandas.pydata.org/docs/development/contributing_codebase.html#contributing-pre-commit Thanks @i1oveMyse1f 