promise.py: make compatible with Twisted-14.0.0

twisted.python.util.unsignedID was removed in Twisted-14.0.0 . We used
it to handle very old pythons (py2.4/py2.5) in which id() sometimes
returned a negative number. This only affects the cosmetic repr() of a
Promise, so this probably doesn't affect 2.4/2.5 compatibility (which is
currently untested).