CLN: standardize string repr to use __repr__

[topper-123]

• xref CLN: remove PY2 #25725 & Remove StringMixin from code base #26495
• tests added / passed
• 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:

        # 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__.