Remove pd_array from tests
- [x] closes https://github.com/MarcoGorelli/pyladies-sprint-March-2021/issues/7
- [x] tests 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
