CLN: standardize string repr to use __repr__
- [x] xref #25725 & #26495
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Removes use of ``StringMixin`` from io.pytables and from io.stata. It's now only used in 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. I'd appreciate any hint.

Because of the above problem, I've changed ``StringMixin`` to only define a ``__repr__`` and making related changes. This means that all string reps in pandas are now defined through ``__repr__``, which is better than defining them through ``__str__``.