Find and warn about unorderable keys in BTrees

[icemac]

BTrees 4.x no longer supports keys which are not orderable.
In Python 3 deleting objects which such a key is not supported.
Thus we should support it during the conversion:

• add an ability to warn about unorderable keys (like None) in BTrees
• add an ability to delete such keys.

See https://groups.google.com/d/msg/zodb/xhVM0ejl6aE/p2nY-1Z7AwAJ

[icemac]

Since the BTrees 4.4.0 this no longer seems to be a problem as it again allows None keys.