CLN: Move base.StringMixin to computations.common
``StringMixin`` should be removed, but I can't figure out how to remove it without breaking tests, so this moves ``StringMixin`` to core.computation, which is the only module, where it is currently used.

This makes ``core.base.py`` a bit cleaner.
Generally +1.  If you were inclined to try to separate out the just-for-`io.pytables` parts I'd offer moral support for that too. I’ve updated the PR. thanks @topper-123 