Python 2: OxBTrees allow types as keys; Python 3 does not

[jamadden]

Consider:

class Old:
    pass

class New(object):
    pass

from BTrees import family64
bt = family64.OO.BTree()

On Python 3, both Old and New, which are types, cannot go in the tree because they have default comparison:

>>> bt[Old] = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Object has default comparison
>>> bt[New] = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Object has default comparison

Under Python 2, however, the default comparison is only detected for old-style classes; new style type apparently doesn't have default comparison that we can detect:

>>> bt[Old] = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Object has default comparison
>>> bt[New] = 1

Type ordering in Python 2 is based on address, making it not suitable for use in a BTree. It emits a warning if you run Python with the -3 argument.

Can we try harder (or just special case) to find that type objects shouldn't be used as keys?