API: silent overflow in Series(bigints, dtype="int8")
- [ ] 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
- [ ] whatsnew entry

This is made complicated by a) #40110 and b) MaskedArray behavior, which i need to check but may be changeable.