ENH: Make ClosedNamespace extend str like Namespace does

Useful for testing URIRef is from a particular namespace
by means of:

    URIRef(...).startswith(NS)
               ^^^^^^^^^^^^^^^

where `URIRef.defrag()` currently wouldn't cut it.

rdflib/namespace.py

@@ -172,18 +172,21 @@ def __repr__(self):
         return "URIPattern(%r)" % str(self)
 
 
-class ClosedNamespace(object):
+class ClosedNamespace(str):
     """
     A namespace with a closed list of members
 
-    Trying to create terms not listen is an error
+    Trying to create terms not listed is an error
     """
 
-    def __init__(self, uri, terms):
-        self.uri = uri
-        self.__uris = {}
-        for t in terms:
-            self.__uris[t] = URIRef(self.uri + t)
+    def __new__(cls, uri, terms):
+        try:
+            rt = super().__new__(cls, uri)
+        except UnicodeDecodeError:
+            rt = super().__new__(cls, uri, "utf-8")
+        rt.uri = uri  # back-compat
+        rt.__uris = {t: URIRef(rt + t) for t in terms}
+        return rt
 
     def term(self, name):
         uri = self.__uris.get(name)
@@ -204,9 +207,6 @@ def __getattr__(self, name):
             except KeyError as e:
                 raise AttributeError(e)
 
-    def __str__(self):
-        return str(self.uri)
-
     def __repr__(self):
         return "rdf.namespace.ClosedNamespace(%r)" % str(self.uri)
 
@@ -222,8 +222,9 @@ class _RDFNamespace(ClosedNamespace):
     Closed namespace for RDF terms
     """
 
-    def __init__(self):
-        super(_RDFNamespace, self).__init__(
+    def __new__(cls):
+        return super().__new__(
+            cls,
             URIRef("http://www.w3.org/1999/02/22-rdf-syntax-ns#"),
             terms=[
                 # Syntax Names

test/test_namespace.py

@@ -111,6 +111,9 @@ def add_not_in_namespace(s):
 
         # a property name within the core FOAF namespace
         self.assertEqual(
-            add_not_in_namespace("givenName"),
+            FOAF.givenName,
             URIRef("http://xmlns.com/foaf/0.1/givenName"),
         )
+
+        # namescape can be used as str
+        self.assertTrue(FOAF.givenName.startswith(FOAF))