CLN changed .format to f-string for test_converter.py, test_datetimel…
…ike.py, test_series.py #29547

https://github.com/pandas-dev/pandas/issues/29547

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