Make _bleio.Attribute be more like an enum, not just int values

[dhalbert]

The the _bleio.Attribute values are plain ints, but https://github.com/adafruit/Adafruit_Blinka_bleio/ uses CPython Enum. This causes problems when trying to add type annotations: adafruit/Adafruit_Blinka_bleio#44.

The Attribute code was written a while ago. I think we could switch it to use py/enum.h, and fix this incompatibility.