Raise deprecation warning only when attribute is accessed

Fixes braintree#148

braintree/dispute.py

@@ -5,7 +5,15 @@
 from braintree.dispute_details import DisputeEvidence, DisputeStatusHistory, DisputePayPalMessage
 from braintree.configuration import Configuration
 
-class Dispute(AttributeGetter):
+
+class _DisputeType(type):
+    @property
+    def ChargebackProtectionLevel(cls):
+        warnings.warn("Use ProtectionLevel enum instead", DeprecationWarning)
+        return cls._ChargebackProtectionLevel
+
+
+class Dispute(AttributeGetter, metaclass=_DisputeType):
     # NEXT_MAJOR_VERSION this can be an enum! they were added as of python 3.4 and we support 3.5+
     class Status(object):
         """
@@ -72,19 +80,22 @@ class Kind(object):
         Retrieval      = "retrieval"
 
     # NEXT_MAJOR_VERSION Remove this enum
-    class ChargebackProtectionLevel(object):
+    class _ChargebackProtectionLevel(object):
         """
         Constants representing dispute ChargebackProtectionLevel. Available types are:
 
         * braintree.Dispute.ChargebackProtectionLevel.EFFORTLESS
         * braintree.Dispute.ChargebackProtectionLevel.STANDARD
         * braintree.Dispute.ChargebackProtectionLevel.NOT_PROTECTED
         """
-        warnings.warn("Use ProtectionLevel enum instead", DeprecationWarning)
         Effortless     = "effortless"
         Standard       = "standard"
         NotProtected   = "not_protected"
 
+    @property
+    def ChargebackProtectionLevel(self):
+        return self.__class__.ChargebackProtectionLevel
+
     # NEXT_MAJOR_VERSION this can be an enum! they were added as of python 3.4 and we support 3.5+
     class PreDisputeProgram(object):
         """