Fix deprecated forms of asset and usage keys

what is E1101? Please use "pylint: disable=name-of-violation".

Also in #13 I was mapping "revision" to "branch" in `CourseLocator` - this conflicts with that - see https://github.com/edx/opaque-keys/pull/13/files#diff-07b3fcc131b415c9137c213811417dfeR641