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

@@ -67,9 +67,8 @@ def test_curie(self):
         with self.assertRaises(ValueError):
             Curie("1df:type")
         self.assertFalse(Curie.is_valid('1df:type'))
-        with self.assertRaises(ValueError):
-            Curie("rdf:17")
-        self.assertFalse(Curie.is_valid('rdf:17'))
+        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))