DEPR: EWM.vol
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

`vol` is undocumented and was just `std` which is more standard in our API
thanks @mroeschke 