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.