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

Hello @alansammarone! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/__init__.py`](https://github.com/alansammarone/pandas/blob/786e43f96d1b7583e8e400f5f89854a9e4c3e523/pandas/__init__.py) !

