is_string_like existed both in matplotlib and matplotlib.cbook

This closes #1019.

``is_string_like`` existed both in ``matplotlib`` and in ``matplotlib.cbook``. The proposed fix was to remove  ``is_string_like`` from ``cbook`` and import the one from ``matplotlib`` in ``cbook``. I'm somewhat unsatisfied with this solution, but backward compatibility is completely maintained, and it is better than what existed before (code duplication).