BUG: do not crash on a callable grouper returning tuples
- [x] closes #22257
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Notice I didn't have time to test which commit caused the regression.
Hello @toobaz! Thanks for updating the PR.





Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on August 12, 2018 at 08:48 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22279?src=pr&el=h1) Report
> Merging [#22279](https://codecov.io/gh/pandas-dev/pandas/pull/22279?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d09db1fe8dc3bc08c5ca0eff97d43d2b30d5233e?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22279/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/22279?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22279      +/-   ##
==========================================
- Coverage   92.08%   92.07%   -0.01%     
==========================================
  Files         169      169              
  Lines       50694    50699       +5     
==========================================
+ Hits        46682    46683       +1     
- Misses       4012     4016       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.48% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.34% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22279?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/grouper.py](https://codecov.io/gh/pandas-dev/pandas/pull/22279/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGVyLnB5) | `97.48% <100%> (-0.69%)` | :arrow_down: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/22279/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.08% <0%> (-0.26%)` | :arrow_down: |
| [pandas/core/internals/managers.py](https://codecov.io/gh/pandas-dev/pandas/pull/22279/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL21hbmFnZXJzLnB5) | `96.58% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22279?src=pr&el=continue).
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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22279?src=pr&el=footer). Last update [d09db1f...9123f7d](https://codecov.io/gh/pandas-dev/pandas/pull/22279?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @toobaz can you merge master and move the doc-note to 0.24.0 closing as stale, but pls reopen if can continue