Replace dicts with OrderedDicts in groupby aggregation functions
- [x] closes #25692
- [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/25693?src=pr&el=h1) Report
> Merging [#25693](https://codecov.io/gh/pandas-dev/pandas/pull/25693?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5c341dc136cae12bf23c1a4b8e34c9951828c0cf?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/25693/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25693?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25693      +/-   ##
==========================================
- Coverage   91.28%   91.28%   -0.01%     
==========================================
  Files         173      173              
  Lines       52965    52965              
==========================================
- Hits        48349    48348       -1     
- Misses       4616     4617       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.86% <100%> (ø)` | :arrow_up: |
| #single | `41.74% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25693?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/25693/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `87% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25693/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.98% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25693?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/25693?src=pr&el=footer). Last update [5c341dc...b472e66](https://codecov.io/gh/pandas-dev/pandas/pull/25693?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/25693?src=pr&el=h1) Report
> Merging [#25693](https://codecov.io/gh/pandas-dev/pandas/pull/25693?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e8d951d2edef5df2cc7bf60e90f345fd5fd18a6b?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/25693/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25693?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25693   +/-   ##
=======================================
  Coverage   91.25%   91.25%           
=======================================
  Files         172      172           
  Lines       52965    52965           
=======================================
  Hits        48335    48335           
  Misses       4630     4630
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <100%> (ø)` | :arrow_up: |
| #single | `41.74% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25693?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/25693/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `87% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25693?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/25693?src=pr&el=footer). Last update [e8d951d...6690b00](https://codecov.io/gh/pandas-dev/pandas/pull/25693?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @alexander-ponomaroff! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-03-18 17:17:00 UTC looks good. can you address @TomAugspurger comment. ping on green. @jreback Done, tests are green. Thanks @alexander-ponomaroff ! No problem @WillAyd :)