BUG: groupby.transform retains timezone information
State: closed

- [x] closes #24198
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


Labels:
name: Bug
name: Groupby
name: Timezones
description: Timezone data dtype

Comments:
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25264?src=pr&el=h1) Report
> Merging [#25264](https://codecov.io/gh/pandas-dev/pandas/pull/25264?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5f73594d49ea06ada4c6779006c00aaf41935595?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25264/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25264?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25264   +/-   ##
=======================================
  Coverage   91.93%   91.93%           
=======================================
  Files         166      166           
  Lines       52187    52187           
=======================================
  Hits        47978    47978           
  Misses       4209     4209
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.48% <100%> (ø)` | :arrow_up: |
| #single | `42.14% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25264?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/25264/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `86.98% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25264?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/25264?src=pr&el=footer). Last update [5f73594...3c5fd19](https://codecov.io/gh/pandas-dev/pandas/pull/25264?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25264?src=pr&el=h1) Report
> Merging [#25264](https://codecov.io/gh/pandas-dev/pandas/pull/25264?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/83fe6ca422ecc35f311897abedadcc492b2a0777?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25264/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25264?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25264   +/-   ##
=======================================
  Coverage   91.72%   91.72%           
=======================================
  Files         173      173           
  Lines       52831    52831           
=======================================
  Hits        48457    48457           
  Misses       4374     4374
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.28% <100%> (ø)` | :arrow_up: |
| #single | `41.72% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25264?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/25264/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `86.98% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25264?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/25264?src=pr&el=footer). Last update [83fe6ca...288aa07](https://codecov.io/gh/pandas-dev/pandas/pull/25264?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

@mroeschke sorry, can you merge master fixup the whatsnew note
thanks!