BUG: at/iat __setitem__ failing to cast
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

thanks. we might have some open issues that this would close if you can search at some point.