A cython __richcmp__ function isn't allowed to assume that its first
argument is the same as the type of the class to which it belongs, so
our code needs to account for either of its two arguments being of the
wrong type.
Furthermore, the correct way for __richcmp__ to handle when it doesn't
know how to do a comparison is to return NotImplemented.
The >= comparison for the Cythonized Security object was actually
doing <=. Fix this and add unit tests for all the Security object rich
comparison operators.