Hide dynamically defined metaclass base from Sphinx.

The inheritance diagram extension in Sphinx 1.2 stores class references,
which Sphinx then tries to pickle. This chokes on our dynamically
defined base classes from the with_metaclass function. Starting the name
with an underscore makes Sphinx ignore the class.

(Bug obscurity/fix simplicity) ratio is high.
Merge pull request #4532 from takluyver/sphinx-1.2-dynamic-metaclasses

Hide dynamically defined metaclass base from Sphinx.