Test to ensure that the ``plib`` abstract container classes are registered
with the appropriate abstract base classes. Note that we cheat for Python
versions earlier than 2.6 (easier than ginning up a conditional unittest
test case). ;-)

    >>> from plib.stdlib import abstractcontainer, abstractsequence, abstractlist, basecontainer, basesequence, baselist
    >>> try:
    ...     from collections import Sequence, MutableSequence
    ... except ImportError:
    ...     Sequence = abstractcontainer
    ...     MutableSequence = abstractlist
    ... 
    >>> issubclass(abstractcontainer, Sequence)
    True
    >>> issubclass(abstractsequence, Sequence)
    True
    >>> issubclass(abstractlist, Sequence)
    True
    >>> issubclass(basecontainer, Sequence)
    True
    >>> issubclass(basesequence, Sequence)
    True
    >>> issubclass(baselist, Sequence)
    True
    >>> issubclass(abstractcontainer, MutableSequence)
    False
    >>> issubclass(abstractsequence, MutableSequence)
    False
    >>> issubclass(abstractlist, MutableSequence)
    True
    >>> issubclass(basecontainer, MutableSequence)
    False
    >>> issubclass(basesequence, MutableSequence)
    False
    >>> issubclass(baselist, MutableSequence)
    True
