Add type annotation to config_init
- [X] closes #26297
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26300?src=pr&el=h1) Report
> Merging [#26300](https://codecov.io/gh/pandas-dev/pandas/pull/26300?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/94c8c94dccfa7934699c314420f0898d81425519?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26300/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26300?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26300      +/-   ##
==========================================
- Coverage   91.98%   91.98%   -0.01%     
==========================================
  Files         175      175              
  Lines       52374    52373       -1     
==========================================
- Hits        48178    48173       -5     
- Misses       4196     4200       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.53% <ø> (-0.01%)` | :arrow_down: |
| #single | `40.72% <ø> (-0.15%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26300?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/config\_init.py](https://codecov.io/gh/pandas-dev/pandas/pull/26300/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnX2luaXQucHk=) | `96.94% <ø> (-0.03%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26300/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26300/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.9% <0%> (-0.12%)` | :arrow_down: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/26300/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `97.98% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26300?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/26300?src=pr&el=footer). Last update [94c8c94...7b363f6](https://codecov.io/gh/pandas-dev/pandas/pull/26300?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. ping on green. @jreback The board is green thanks @gwrome 