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
    >>> from plib.stdlib import basecontainer, basesequence, baselist
    >>> from plib.stdlib import abstractkeyed, abstractmapping, abstractdict
    >>> from plib.stdlib import basekeyed, basemapping, basedict
    >>> try:
    ...     from collections import Sequence, MutableSequence, Mapping, MutableMapping
    ... except ImportError:
    ...     Sequence = abstractcontainer
    ...     MutableSequence = abstractlist
    ...     Mapping = abstractkeyed
    ...     MutableMapping = abstractdict
    ... 
    >>> 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
    >>> issubclass(abstractkeyed, Mapping)
    True
    >>> issubclass(abstractmapping, Mapping)
    True
    >>> issubclass(abstractdict, Mapping)
    True
    >>> issubclass(basekeyed, Mapping)
    True
    >>> issubclass(basemapping, Mapping)
    True
    >>> issubclass(basedict, Mapping)
    True
    >>> issubclass(abstractkeyed, MutableMapping)
    False
    >>> issubclass(abstractmapping, MutableMapping)
    False
    >>> issubclass(abstractdict, MutableMapping)
    True
    >>> issubclass(basekeyed, MutableMapping)
    False
    >>> issubclass(basemapping, MutableMapping)
    False
    >>> issubclass(basedict, MutableMapping)
    True
