When cls is an ABCMeta, every call to isinstance(x, cls)
records type(x) in the cls._abc_cache of cls_abc_negative_cache.
So we clear these caches at the end of the test.

inspect.isabstract() is not the correct test for all ABCs, because there is no @abstractmethod in io.py (why?)
isinstance(cls, ABCMeta) would be more exact, but it fails with an infinite recursion.
So I used a hack to determine whether a class is an ABCMeta.

The true correction would be to turn cls._abc_cache &co into a WeakSet, as py3k does.
But classic classes are not weak referenceable...

Of course, this change should not be merged into the py3k branch.