FIX `groupby` with column selection not returning tuple when grouping by list of a single element
- [x] Closes #53500
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests)
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit)
- [x] Added an entry in the latest `doc/source/whatsnew/v2.1.0.rst` file
@Charlie-XIAO now that #53519 has been merged, the issues with `nth` should be fixed. Can you merge main. > @Charlie-XIAO now that #53519 has been merged, the issues with `nth` should be fixed. Can you merge main.

Sure, will do. Thanks @Charlie-XIAO!