TYP: core/dtypes/cast.py
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

cc @simonjayhawkins  @simonjayhawkins i fyou can have a quick look and merge if good Thanks @arw2019 lgtm one more comment https://github.com/pandas-dev/pandas/pull/37024#discussion_r503495620 ping when green thanks @arw2019 