Issue 18772: Restore set dummy object back to unicode and restore the identity check...

Issue 18772:  Restore set dummy object back to unicode and restore the identity checks in lookkey().

The Gdb prettyprint plugin depended on the dummy object being displayable.
Other solutions besides a unicode object are possible.  For now, get it
back up and running.

The identity checks in lookkey() need to be there to prevent the dummy
object from leaking through Py_RichCompareBool() into user code in the
rare circumstance where the dummy's hash value exactly matches the hash
value of the actual key being looked up.