test: fix wrong expectation on curies

CURIE relative_ref part can perfectly be a number. Fixing wrong
expectation rejecting number only relative_refs.

Signed-off-by: Silvano Cirujano Cuesta <silvano.cirujano-cuesta@siemens.com>

Author: Silvanoc

tests/test_utils/test_metamodelcore.py

@@ -80,6 +80,7 @@ def test_curie(self):
             Curie("1df:type")
         self.assertFalse(Curie.is_valid("1df:type"))
         self.assertTrue(Curie.is_valid("rdf:17"))
+        self.assertTrue(Curie.is_valid("17"))
         nsm = Namespaces(Graph())
         self.assertEqual(RDF.type, Curie("rdf:type").as_uri(nsm))
         self.assertIsNone(Curie("ex:foo").as_uri(nsm))