TYP: add type annotation to expr.py #36027
- [x] closes #36027
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`


> Thanks @VirosaLi for the PR.
> 
> This may be fixed in open PR. see [#36012 (review)](https://github.com/pandas-dev/pandas/pull/36012#pullrequestreview-478726562)

Cool, I'll close this one.