#2503 make singletons compared with "is" not == or !=
Thanks to Wummel for the patch
diff --git a/Lib/bsddb/dbshelve.py b/Lib/bsddb/dbshelve.py
index f5f8b9a..96f604a 100644
--- a/Lib/bsddb/dbshelve.py
+++ b/Lib/bsddb/dbshelve.py
@@ -133,7 +133,7 @@
 
 
     def keys(self, txn=None):
-        if txn != None:
+        if txn is not None:
             return self.db.keys(txn)
         else:
             return self.db.keys()
@@ -157,7 +157,7 @@
 
 
     def items(self, txn=None):
-        if txn != None:
+        if txn is not None:
             items = self.db.items(txn)
         else:
             items = self.db.items()
@@ -168,7 +168,7 @@
         return newitems
 
     def values(self, txn=None):
-        if txn != None:
+        if txn is not None:
             values = self.db.values(txn)
         else:
             values = self.db.values()