BUG: Groupby.cummin/max DataError on datetimes (#15561)
 - [x] closes #15561
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``

`numeric_only=False` set by default for `groupby.cummin/cummax` like `groupby.max/min` allows datetimes to work. 
lgtm. ping on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15569?src=pr&el=h1) Report
> Merging [#15569](https://codecov.io/gh/pandas-dev/pandas/pull/15569?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5f0b69aee3622eed9392cef163e4b31ba742498e?src=pr&el=desc) will **decrease** coverage by `-0.03%`.
> The diff coverage is `100%`.


```diff
@@            Coverage Diff             @@
##           master   #15569      +/-   ##
==========================================
- Coverage   91.05%   91.03%   -0.03%     
==========================================
  Files         137      137              
  Lines       49291    49283       -8     
==========================================
- Hits        44884    44863      -21     
- Misses       4407     4420      +13
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15569?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/compare/5f0b69aee3622eed9392cef163e4b31ba742498e...5e5d70d43edb02d3de1ca31968fa9f933a904da2?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `94.99% <100%> (ø)` | :white_check_mark: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/5f0b69aee3622eed9392cef163e4b31ba742498e...5e5d70d43edb02d3de1ca31968fa9f933a904da2?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :x: |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/compare/5f0b69aee3622eed9392cef163e4b31ba742498e...5e5d70d43edb02d3de1ca31968fa9f933a904da2?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `91.78% <0%> (-0.35%)` | :x: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/5f0b69aee3622eed9392cef163e4b31ba742498e...5e5d70d43edb02d3de1ca31968fa9f933a904da2?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.02% <0%> (-0.34%)` | :x: |
| [pandas/tseries/period.py](https://codecov.io/gh/pandas-dev/pandas/compare/5f0b69aee3622eed9392cef163e4b31ba742498e...5e5d70d43edb02d3de1ca31968fa9f933a904da2?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGVyaW9kLnB5) | `92.67% <0%> (-0.13%)` | :x: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/5f0b69aee3622eed9392cef163e4b31ba742498e...5e5d70d43edb02d3de1ca31968fa9f933a904da2?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.83% <0%> (-0.1%)` | :x: |
| [pandas/tseries/index.py](https://codecov.io/gh/pandas-dev/pandas/compare/5f0b69aee3622eed9392cef163e4b31ba742498e...5e5d70d43edb02d3de1ca31968fa9f933a904da2?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvaW5kZXgucHk=) | `95.32% <0%> (-0.1%)` | :x: |
| [pandas/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/compare/5f0b69aee3622eed9392cef163e4b31ba742498e...5e5d70d43edb02d3de1ca31968fa9f933a904da2?src=pr&el=tree#diff-cGFuZGFzL2luZGV4ZXMvYmFzZS5weQ==) | `96.23% <0%> (ø)` | :white_check_mark: |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15569?src=pr&el=footer). Last update [5f0b69a...5e5d70d](https://codecov.io/gh/pandas-dev/pandas/compare/5f0b69aee3622eed9392cef163e4b31ba742498e...5e5d70d43edb02d3de1ca31968fa9f933a904da2?src=pr&el=footer&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). Thanks!