offsetvector: `cmp` gone in Python 3

The offsetvector class implements the __cmp__ method to define behavior when calling cmp on two instances. However, __cmp__ and cmp are gone in Python 3.

The obvious solution would be to implement __eq__ instead of __cmp__. What's tricky is that the docstring for offsetvector.__cmp__ specifically states that for instances of this class cmp(a, b) and a == b behave differently. So I'm not sure what to do.

This will need to be fixed before the unit tests will pass under Python 3.