CLN: ASV panel_methods
Flake8'd, `param`'d, and this benchmark was timing out on my machine so I scaled down the size of the Panel but should be representative of larger Panels

```
asv dev -b ^panel_methods
· Discovering benchmarks
· Running 2 total benchmarks (1 commits * 1 environments * 2 benchmarks)
[  0.00%] ·· Building for existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[  0.00%] ·· Benchmarking existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[ 50.00%] ··· Running panel_methods.PanelMethods.time_pct_change                                                                          ok
[ 50.00%] ···· 
               ======= =======
                 axis         
               ------- -------
                items   1.54s 
                major   1.39s 
                minor   1.41s 
               ======= =======

[100.00%] ··· Running panel_methods.PanelMethods.time_shift                                                                               ok
[100.00%] ···· 
               ======= =======
                 axis         
               ------- -------
                items   397μs 
                major   385μs 
                minor   390μs 
               ======= =======
``` 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18907?src=pr&el=h1) Report
> Merging [#18907](https://codecov.io/gh/pandas-dev/pandas/pull/18907?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/316acbf065f5d1e30fdc7fcced3a53a2f4b14655?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18907/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18907?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18907      +/-   ##
==========================================
+ Coverage   91.65%   91.67%   +0.02%     
==========================================
  Files         154      154              
  Lines       51368    51368              
==========================================
+ Hits        47080    47092      +12     
+ Misses       4288     4276      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.54% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.84% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18907?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18907/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/18907?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/18907?src=pr&el=footer). Last update [316acbf...456f900](https://codecov.io/gh/pandas-dev/pandas/pull/18907?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks!