BUG: groupby sum, mean, var should always be floats
- [x] closes #41137
- [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

Built on top of #40790
looks good. can you add a whatsnew note. might consider a sub-section as this is a 'bigger' bug fix but either way. @jreback Thanks - whatsnew section added. thanks @rhshadrach 