Initialize cython.const if not present

Add check for cython.const attribute and initialize it.

Author: saipenikalapati

docs/source/conf.py

@@ -30,7 +30,8 @@
     cython.tuple = tuple
 if not hasattr(cython, 'list'):
     cython.list = list
-
+if not hasattr(cython, 'const'):
+    cython.const = object()
 
 class _CImportStub(ModuleType):
     def __getattr__(self, name):