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)

ok so other PR merged, you can rebase
 FYI windows test https://ci.appveyor.com/project/Komnomnomnom/pandas/build/1.0.4 . Thanks for the tip!
 @Komnomnomnom gr8!. note that appveyor is REALLY SLOW!
 @jreback build clean (apart from msgpack & pickle again)
https://travis-ci.org/Komnomnomnom/pandas/builds/126098431

also windows is ok:
https://ci.appveyor.com/project/Komnomnomnom/pandas/build/1.0.8
 just remember how to fix the tag issue.

on your local master
`git push yourorigin master --tags`

tags come from your fork (and not from the clone, pandas)

next time you branch you should be good 
 thanks!

make an issue about the deprecations for 0.19.0 pls.
