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Fix hashability on Python 3

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"Minimum" and "Maximum" Objects
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The ``peak.util.extremes`` module provides a production-quality implementation
of the ``Min`` and ``Max`` objects from PEP 326.  While PEP 326 was rejected
for inclusion in the language or standard library, the objects described in it
are useful in a variety of applications.  In PEAK, they have been used to
implement generic functions (in RuleDispatch and PEAK-Rules), as well as to
handle scheduling and time operations in the Trellis.  Because this has led to
each project copying the same code, we've now split the module out so it can
be used independently.

Some simple usage examples::

    >>> from peak.util.extremes import Min, Max
    >>> try:
    ...    from sys import maxint
    ... except ImportError:
    ...    maxint = (1<<64)

    >>> Min < -maxint
    True
    >>> Min < None
    True
    >>> Min < ''
    True
    >>> Max > maxint
    True
    >>> Max > 99999999999999999
    True

    >>> type(Min)
    <class 'peak.util.extremes.Extreme'>

The ``Min`` object compares less than any other object but itself, while the
``Max`` object compares greater than any other object but itself.  Both are
instances of the ``Extreme`` type, are hashable, and can be used as
dictionary keys:

    >>> x = {Min: 1, Max: 2}

While the original PEP 326 implementation of these extreme values is shorter
than the version used here, it contains a flaw: it does not correctly handle
comparisons with classic class instances.  Therefore, this version defines
methods for all six rich comparison operators, to ensure correct support for
classic as well as new-style classes::

    >>> Max >= Min and Max > Min and Max==Max and Max!=Min
    True
    >>> Max < Min or Max <= Min or Max==Min or Max!=Max
    False

    >>> Min <= Max and Min < Max and Min==Min and Min!=Max
    True
    >>> Min > Max or Min >= Max or Min==Max or Min!=Min
    False

    >>> class X:
    ...     """Ensure rich comparisons work correctly with classic classes"""

    >>> x = X()

    >>> Min<x<Max and Min<=x<=Max and Min!=x!=Max and Max!=x!=Min
    True
    
    >>> Min>x or x>Max or x<=Min or x>=Max or x==Min or Min==x
    False