In the past, an object's tp_compare could return any value.  In 2.2
the docs were tightened to require it to return -1, 0 or 1; and -1 for
an error.

We now issue a warning if the value is not in this range.  When an
exception is raised, we allow -1 or -2 as return value, since -2 will
the recommended return value for errors in the future.  (Eventually
tp_compare will also be allowed to return +2, to indicate
NotImplemented; but that can only be implemented once we know all
extensions return a value in [-2...1].  Or perhaps it will require the
type to set a flag bit.)

I haven't decided yet whether to backport this to 2.2.x.  The patch
applies fine.  But is it fair to start warning in 2.2.2 about code
that worked flawlessly in 2.2.1?