CLN: replace %s syntax with .format in core/indexing.py
Progress toward issue #16130. Converted old string formatting to new string formatting in core/indexing.py.

- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=h1) Report
> Merging [#17357](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/473a7f3c186f6b0bfd9d3ce413fb627cf7a8f111?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `66.66%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17357/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17357      +/-   ##
==========================================
- Coverage   91.03%   90.99%   -0.05%     
==========================================
  Files         162      162              
  Lines       49567    49567              
==========================================
- Hits        45125    45104      -21     
- Misses       4442     4463      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <66.66%> (-0.03%)` | :arrow_down: |
| #single | `40.24% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.94% <66.66%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17357?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/17357?src=pr&el=footer). Last update [473a7f3...6ca062d](https://codecov.io/gh/pandas-dev/pandas/pull/17357?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/17357?src=pr&el=h1) Report
> Merging [#17357](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/473a7f3c186f6b0bfd9d3ce413fb627cf7a8f111?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `66.66%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17357/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17357      +/-   ##
==========================================
- Coverage   91.03%   90.99%   -0.05%     
==========================================
  Files         162      162              
  Lines       49567    49567              
==========================================
- Hits        45125    45104      -21     
- Misses       4442     4463      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <66.66%> (-0.03%)` | :arrow_down: |
| #single | `40.24% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.94% <66.66%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17357?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/17357?src=pr&el=footer). Last update [473a7f3...6ca062d](https://codecov.io/gh/pandas-dev/pandas/pull/17357?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!