CLN: remove StringMixin from code base, except in core.computation
- [x] xref #25725 & #26495
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Removes use of ``StringMixin`` from pandas.io StringMixin is now only used in pandas.core.computation.

I haven't been able to remove this class from core.computation. If I do, I get an error about setting an attribute:

```python
        # assumes that resolvers are going from outermost scope to inner
        if isinstance(local_dict, Scope):
            resolvers += tuple(local_dict.resolvers.maps)
        self.resolvers = DeepChainMap(*resolvers)
E       AttributeError: 'Scope' object has no attribute 'resolvers'

pandas\core\computation\scope.py:134: AttributeError
```

I've looked into this and can't understand why this happens. From the looks of it, I should be able to set that atribute here, but it jusr doesn't work. I'd appreciate any hint.

I'll keep loking into removing it completely, but don't know if I'll crack that part.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26523?src=pr&el=h1) Report
> Merging [#26523](https://codecov.io/gh/pandas-dev/pandas/pull/26523?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ba48fc4a033f11513fa2dd44c946e18b7bc27ad2?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26523/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26523?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26523      +/-   ##
==========================================
- Coverage   91.75%   91.75%   -0.01%     
==========================================
  Files         174      174              
  Lines       50665    50641      -24     
==========================================
- Hits        46489    46465      -24     
  Misses       4176     4176
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.29% <100%> (+0.03%)` | :arrow_up: |
| #single | `41.69% <100%> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26523?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/26523/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `90.29% <100%> (+0.07%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26523/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/26523/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26523?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/26523?src=pr&el=footer). Last update [ba48fc4...0ee15d2](https://codecov.io/gh/pandas-dev/pandas/pull/26523?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/26523?src=pr&el=h1) Report
> Merging [#26523](https://codecov.io/gh/pandas-dev/pandas/pull/26523?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d2beaf3c821236e8e2f6fb0bdaf90ff3d404cc11?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26523/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26523?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26523      +/-   ##
==========================================
- Coverage   91.76%   91.76%   -0.01%     
==========================================
  Files         174      174              
  Lines       50653    50629      -24     
==========================================
- Hits        46482    46458      -24     
  Misses       4171     4171
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <100%> (+0.03%)` | :arrow_up: |
| #single | `41.68% <100%> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26523?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/26523/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `90.29% <100%> (+0.07%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26523/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/26523/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26523?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/26523?src=pr&el=footer). Last update [d2beaf3...aab3dea](https://codecov.io/gh/pandas-dev/pandas/pull/26523?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @topper-123 

not real sure about that error you are getting above