CLN: replace %s syntax with .format in core.panel
- [x] Progress towards #16130 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Note, there's still a couple usages in this file in the `Appender` decorators, I'm planning on going through all `appender`/`shared_docs` usages in the near future to clean these up too.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=h1) Report
> Merging [#18315](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fe1bfd770f203acdbb8dbb2ccfcffa8e1c6214fc?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `72.72%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18315/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18315      +/-   ##
==========================================
- Coverage   91.37%   91.36%   -0.01%     
==========================================
  Files         164      164              
  Lines       49854    49807      -47     
==========================================
- Hits        45553    45508      -45     
+ Misses       4301     4299       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.17% <72.72%> (+0.01%)` | :arrow_up: |
| #single | `39.47% <18.18%> (-0.03%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.14% <72.72%> (-0.15%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `96.96% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.25% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18315?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/18315?src=pr&el=footer). Last update [fe1bfd7...25c2008](https://codecov.io/gh/pandas-dev/pandas/pull/18315?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/18315?src=pr&el=h1) Report
> Merging [#18315](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fe1bfd770f203acdbb8dbb2ccfcffa8e1c6214fc?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `72.72%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18315/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18315      +/-   ##
==========================================
- Coverage   91.37%   91.36%   -0.01%     
==========================================
  Files         164      164              
  Lines       49854    49807      -47     
==========================================
- Hits        45553    45508      -45     
+ Misses       4301     4299       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.17% <72.72%> (+0.01%)` | :arrow_up: |
| #single | `39.47% <18.18%> (-0.03%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.14% <72.72%> (-0.15%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `96.96% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.25% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18315?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/18315?src=pr&el=footer). Last update [fe1bfd7...25c2008](https://codecov.io/gh/pandas-dev/pandas/pull/18315?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/18315?src=pr&el=h1) Report
> Merging [#18315](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fe1bfd770f203acdbb8dbb2ccfcffa8e1c6214fc?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `72.72%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18315/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18315      +/-   ##
==========================================
- Coverage   91.37%   91.36%   -0.01%     
==========================================
  Files         164      164              
  Lines       49854    49807      -47     
==========================================
- Hits        45553    45508      -45     
+ Misses       4301     4299       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.17% <72.72%> (+0.01%)` | :arrow_up: |
| #single | `39.47% <18.18%> (-0.03%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.14% <72.72%> (-0.15%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `96.96% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.25% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18315?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/18315?src=pr&el=footer). Last update [fe1bfd7...25c2008](https://codecov.io/gh/pandas-dev/pandas/pull/18315?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!