Fix read_json category dtype
- [X] closes #21892
- [ ] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] Add support for "category" dtype in read_json

Hello @taoufik07! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-01-06 12:30:52 UTC Hi @taoufik07  - sorry to chase you up, just wanted to ask whether you're still working on this :) Hey @MarcoGorelli, sorry I'm kinda busy right now, yes I would love to keep working on this, I'll try finish the tests in the couple days Thanks - no hurry, just checking it's not gone stale @taoufik07 closing as stale. ping if you want to continue.