CLN: Misc Python 2 references
- [x] xref https://github.com/pandas-dev/pandas/issues/25725#issuecomment-482460520
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

The notable change here is removing the SIX license since I think we've removed any borrowed compatibility code. 