fix bug when combining groupby with resample and interpolate with dat…
…etime-index (GH 35325)

- [x] closes #35325
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Thanks @CloseChoice for the PR. The test failures should be fixed on master. can you merge upstream/master. > Thanks @CloseChoice for the PR. The test failures should be fixed on master. can you merge upstream/master.

Done, though I am not quite sure if my whatsnew entry is correct. There seemed to be some problems with building the documentation. I also had a similar issue on another PR. thanks @CloseChoice very nice