Add failover-state as a valid ltype

The previous commit which added lookup of failover-state object attributes was missing this.

pypureomapi.py

@@ -1238,7 +1238,7 @@ def __lookup(self, ltype, **kwargs):
 		@raises socket.error:
 		"""
 		ltype_utf = ltype.encode("utf-8")
-		assert ltype_utf in [b"host", b"lease"]
+		assert ltype_utf in [b"host", b"lease", b"failover-state"]
 		msg = OmapiMessage.open(ltype_utf)
 		for k in kwargs:
 			if k == "raw":