C BTrees allow some unorderable types

[jamadden]

Given this class:

class Bad(object):
    def __eq__(self, other):
        return False

Under Python 2.7, you can use instances of this class as a key in any size C BTree.

Under Python 3, you can insert this into an empty tree only. Once you do, the tree is broken. Jim Fulton suggested that the key should be compared to itself on an empty tree to catch this (but given that pointer equality is checked first, it's not clear exactly how that would be done).

The Python implementation rejects this object in all versions in all size trees.

See https://groups.google.com/forum/#!topic/zodb/xhVM0ejl6aE