DEPR/BUG: fix Series.argmin/max
State: closed

 - [x] closes #16830
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [ ] whatsnew entry

This PR actually implements the proper behavior for argmax() and argmin() for both Series and DataFrame. We will have to wait to rebase this onto master until after a new release.

Labels:
name: Bug
name: Deprecate
description: Functionality to remove in pandas

Comments:
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=h1) Report
> Merging [#16964](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/96168ef698ac8bbccba251258ee66958359b11bf?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16964/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16964      +/-   ##
==========================================
+ Coverage   90.98%   90.99%   +<.01%     
==========================================
  Files         161      161              
  Lines       49288    49300      +12     
==========================================
+ Hits        44846    44861      +15     
+ Misses       4442     4439       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.76% <100%> (+0.02%)` | :arrow_up: |
| #single | `40.19% <26.31%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <100%> (-0.1%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.91% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.05% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16964?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/16964?src=pr&el=footer). Last update [96168ef...a71b040](https://codecov.io/gh/pandas-dev/pandas/pull/16964?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/16964?src=pr&el=h1) Report
> Merging [#16964](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7ffe7fc21f3dc4ca444de9c83dbf61313b6986e2?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16964/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16964      +/-   ##
==========================================
- Coverage   90.99%   90.97%   -0.02%     
==========================================
  Files         161      161              
  Lines       49290    49302      +12     
==========================================
+ Hits        44851    44854       +3     
- Misses       4439     4448       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.75% <100%> (ø)` | :arrow_up: |
| #single | `40.19% <26.31%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <100%> (-0.1%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.91% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16964?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/16964?src=pr&el=footer). Last update [7ffe7fc...5f628c2](https://codecov.io/gh/pandas-dev/pandas/pull/16964?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

Tagging this for `1.0` unless we are doing another 0.2x version after 0.21
I would rather open an issue that references this and mark it for 1.0. Then the PR can be resurrected at that time.