Added cast blacklist for certain transform agg funcs
- [X] closes #19200
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19355?src=pr&el=h1) Report
> Merging [#19355](https://codecov.io/gh/pandas-dev/pandas/pull/19355?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9872d6757e5117dce070981141cee562f675694e?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19355/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19355?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19355      +/-   ##
==========================================
+ Coverage   91.54%   91.57%   +0.02%     
==========================================
  Files         150      150              
  Lines       48702    48705       +3     
==========================================
+ Hits        44585    44600      +15     
+ Misses       4117     4105      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.94% <100%> (+0.02%)` | :arrow_up: |
| #single | `41.72% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19355?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/19355/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.15% <100%> (+0.01%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19355/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19355?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/19355?src=pr&el=footer). Last update [9872d67...3f8b7f3](https://codecov.io/gh/pandas-dev/pandas/pull/19355?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @WillAyd nice patch! keep em coming!