BUG: GH15426 timezone lost in groupby-agg with cython functions
 - [x] closes #15426
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15433?src=pr&el=h1) Report
> Merging [#15433](https://codecov.io/gh/pandas-dev/pandas/pull/15433?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b3ae4c7698de4623e1279d579b46192ef79250d1?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.


```diff
@@            Coverage Diff             @@
##           master   #15433      +/-   ##
==========================================
+ Coverage   90.36%   90.37%   +<.01%     
==========================================
  Files         136      136              
  Lines       49553    49554       +1     
==========================================
+ Hits        44780    44782       +2     
+ Misses       4773     4772       -1
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15433?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/types/cast.py](https://codecov.io/gh/pandas-dev/pandas/compare/b3ae4c7698de4623e1279d579b46192ef79250d1...64a84cac5dc88a34466dec09cb1384cb327875b2?src=pr&el=tree#diff-cGFuZGFzL3R5cGVzL2Nhc3QucHk=) | `85.45% <100%> (+0.02%)` | :white_check_mark: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/compare/b3ae4c7698de4623e1279d579b46192ef79250d1...64a84cac5dc88a34466dec09cb1384cb327875b2?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.31% <0%> (ø)` | :white_check_mark: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/b3ae4c7698de4623e1279d579b46192ef79250d1...64a84cac5dc88a34466dec09cb1384cb327875b2?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.36% <0%> (+0.33%)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15433?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15433?src=pr&el=footer). Last update [b3ae4c7...64a84ca](https://codecov.io/gh/pandas-dev/pandas/compare/b3ae4c7698de4623e1279d579b46192ef79250d1...64a84cac5dc88a34466dec09cb1384cb327875b2?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). add examples from https://github.com/pandas-dev/pandas/issues/10668 as well. (and you can list list issue in the whatsnew too) can you update thanks!