SF 563203. Replaced 'has_key()' with 'in'.
diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index 853949b..7afff50 100755
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -265,7 +265,7 @@
     args = sys.argv[1:]
     if not args:
         for key in 'MAILDIR', 'MAIL', 'LOGNAME', 'USER':
-            if os.environ.has_key(key):
+            if key in os.environ:
                 mbox = os.environ[key]
                 break
         else: