support 'frozenset' in cython namespace (just like 'set')

Cython/Compiler/ParseTreeTransforms.py

@@ -1261,8 +1261,8 @@ class TransformBuiltinMethods(EnvTransform):
                 node = BoolNode(node.pos, value=True)
             elif attribute == u'NULL':
                 node = NullNode(node.pos)
-            elif attribute == u'set':
-                node = NameNode(node.pos, name=EncodedString('set'))
+            elif attribute in (u'set', u'frozenset'):
+                node = NameNode(node.pos, name=EncodedString(attribute))
             elif not PyrexTypes.parse_basic_type(attribute):
                 error(node.pos, u"'%s' not a valid cython attribute or is being used incorrectly" % attribute)
         return node

Cython/Shadow.py

@@ -162,14 +162,14 @@ except NameError: # Py3
 
 try:
     # Python 3
-    from builtins import set
+    from builtins import set, frozenset
 except ImportError:
     try:
         # Python 2.4+
-        from __builtin__ import set
+        from __builtin__ import set, frozenset
     except ImportError:
         # Py 2.3
-        from sets import Set as set
+        from sets import Set as set, ImmutableSet as frozenset
 
 # Predefined types