CLN: replace %s syntax with .format in pandas.core: categorical, common, config, config_init
Progress towards #16130

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

Replaced `%s` syntax with `.format` in pandas.core: `categorical.py`, `common.py`, `config.py`, `config_init.py`. Additionally, made some of the existing positional `.format` code more explicit.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=h1) Report
> Merging [#17735](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/408ecd21759979f94952a48a0e4a2b2a608ee84d?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `77.77%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17735/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17735      +/-   ##
==========================================
- Coverage   91.23%   91.23%   -0.01%     
==========================================
  Files         163      163              
  Lines       49796    49813      +17     
==========================================
+ Hits        45430    45445      +15     
- Misses       4366     4368       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.02% <77.77%> (+0.01%)` | :arrow_up: |
| #single | `40.32% <9.25%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/config\_init.py](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnX2luaXQucHk=) | `96.12% <100%> (+0.06%)` | :arrow_up: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.42% <42.85%> (-0.56%)` | :arrow_down: |
| [pandas/core/config.py](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnLnB5) | `87.7% <78.57%> (-0.39%)` | :arrow_down: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `95.59% <87.5%> (-0.12%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.2% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17735?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/17735?src=pr&el=footer). Last update [408ecd2...687be9a](https://codecov.io/gh/pandas-dev/pandas/pull/17735?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/17735?src=pr&el=h1) Report
> Merging [#17735](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/408ecd21759979f94952a48a0e4a2b2a608ee84d?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `77.77%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17735/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17735      +/-   ##
==========================================
- Coverage   91.23%    91.2%   -0.03%     
==========================================
  Files         163      163              
  Lines       49796    49813      +17     
==========================================
+ Hits        45430    45433       +3     
- Misses       4366     4380      +14
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89% <77.77%> (-0.01%)` | :arrow_down: |
| #single | `40.32% <9.25%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/config\_init.py](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnX2luaXQucHk=) | `96.12% <100%> (+0.06%)` | :arrow_up: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.42% <42.85%> (-0.56%)` | :arrow_down: |
| [pandas/core/config.py](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnLnB5) | `87.7% <78.57%> (-0.39%)` | :arrow_down: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `95.59% <87.5%> (-0.12%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17735?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/17735?src=pr&el=footer). Last update [408ecd2...687be9a](https://codecov.io/gh/pandas-dev/pandas/pull/17735?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jschendel 