ENH: Accept list as level for groupby in non-MultiIndexed objects
- [x] closes #13901
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13907?src=pr) is 85.25% (diff: 100%)

> Merging [#13907](https://codecov.io/gh/pydata/pandas/pull/13907?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13907   diff @@
==========================================
  Files           140        140          
  Lines         50568      50575     +7   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43112      43119     +7   
  Misses         7456       7456          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [a7469cf...fcbb724](https://codecov.io/gh/pydata/pandas/compare/a7469cf98275a183ad2e4bfafa9706a1ef8d035e...fcbb72402f0cf4da1d2ccfdda39ac62a1175fa35?src=pr)
 @sinhrks Made all changes and additions.
 @jreback @sinhrks Check out the latest amendment. How does it look to you?
 @jreback There is a test that fails, but it has nothing to do with `groupby` and I cannot reproduce it on my machine. Thoughts?
 @jreback
 can you rebase / update?
 Rebased and all green.
 ok lgtm. @jorisvandenbossche 
 thanks!
