MAINT: Enforce string type for where parameter
Deprecated in 0.11.0.

xref #12027.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15798?src=pr&el=h1) Report
> Merging [#15798](https://codecov.io/gh/pandas-dev/pandas/pull/15798?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9d3554c26aa85d66cbfe6f481464f1a357af5e12?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `85.71%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15798/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/15798?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15798      +/-   ##
==========================================
- Coverage   91.01%      91%   -0.01%     
==========================================
  Files         143      143              
  Lines       49400    49370      -30     
==========================================
- Hits        44963    44931      -32     
- Misses       4437     4439       +2
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15798?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/computation/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/15798?src=pr&el=tree#diff-cGFuZGFzL2NvbXB1dGF0aW9uL3B5dGFibGVzLnB5) | `92.35% <85.71%> (+1.59%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/15798?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/15798?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.96% <0%> (-0.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/15798?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.86% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15798?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/15798?src=pr&el=footer). Last update [9d3554c...06adda1](https://codecov.io/gh/pandas-dev/pandas/pull/15798?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). lgtm. can you add a deprecation removal note. ping when pushed. also this *might* still be used in the doc-build (though I think I took most of this out). can you check? @jreback : I don't see any AFAICT. thanks @gfyoung 