BUG: Issue #29128 Series.var not returning the correct result
- [ x ] closes #29128
- [ x ] tests added / passed
- [ x ] passes `black pandas`
- [ x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ x ] whatsnew entry

This PR closes https://github.com/pandas-dev/pandas/issues/29128