WIP: CI: Setup 3.9 Travis Build
- [x] ref https://github.com/pandas-dev/pandas/issues/33395

- In a very similar fashion to @jbrockmendel work for 3.8 - might be useful to have start a 3.9 build so we can start eliminating the issues @TomAugspurger mentioned in https://github.com/pandas-dev/pandas/issues/33395