BUG: json invoke default handler for unsupported numpy dtypes
- [x] closes #12554
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry
- [x] valgrind clean
- [ ] windows tests ok
- [x] vbench ok

Recommend to merge after #12802 has been accepted. (valgrind should be clean then)
