CI: py38 builds
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

greenish hmm does this fix? > hmm does this fix?

fixes the py38 issue yah btw who is the go-to person for merge_asof? you, me or @mroeschke  hah if its me then we're in trouble