Define DataFrame plot methods in DataFrame
instead of pinning them on at the bottom of the module.

This is a follow-up to <s>#16391</s>#16931 with a smaller scope, avoiding the as-yet-unresolved circular import problem.

 - [x] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17020?src=pr&el=h1) Report
> Merging [#17020](https://codecov.io/gh/pandas-dev/pandas/pull/17020?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/aead041fece0ef17a81218585329109e17b5deb9?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/17020/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17020?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17020      +/-   ##
==========================================
- Coverage      91%   90.98%   -0.02%     
==========================================
  Files         161      161              
  Lines       49305    49303       -2     
==========================================
- Hits        44868    44857      -11     
- Misses       4437     4446       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.75% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.19% <22.22%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17020?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/17020?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.8% <100%> (+0.07%)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17020?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <100%> (-0.11%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17020?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/sparse/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/17020?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL3Nlcmllcy5weQ==) | `95.06% <0%> (-0.02%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/17020?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.91% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17020?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/17020?src=pr&el=footer). Last update [aead041...f8f46fa](https://codecov.io/gh/pandas-dev/pandas/pull/17020?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I think the issue reference at the top is incorrect, can you fix. thanks!