Undefined warning variable

[Polandia94]

The file payment_method_gateway.py is used warnings.warn but not importing warning, this will result on a runtime error if the code is readed. As this is under a # NEXT_MAJOR_VERSION remove these checks when the attributes are removed probablly better to remove the attributes than to add the import

braintree_python/braintree/payment_method_gateway.py

Line 158 in a16c113

warnings.warn("device_session_id is deprecated, use device_data parameter instead", DeprecationWarning)