weboob.capabilities.base: use int.__str__ as IntEnum.__str__ for compat
Python 2's json module stringifies values using the builtin str() function, so any custom __str__ function will be used instead of the number representation: json.dumps([AccountType.CHECKING]) will then return invalid JSON "[CHECKING]" instead of "[1]". By enforcing use of int.__str__, the previous behavior is restored. See https://bugs.python.org/issue18264 (it's fixed in Python 3).