Make inline "pylint: disable=" notes more clear

Switch ambiguous statements such as "pylint: disable=W0621" with clearer disable statements such as "pylint: disable=redefined-outer-name". I propose that we should be using the clearer form throughout the codebase, so code reviewers know what's going on and coders don't blindly carry over disable statements that no longer apply.

This is also the recommendation made here: https://github.com/edx/edx-platform/blob/master/docs/en_us/developers/source/testing/code-quality.rst
Worth mentioning, some of our pylint errors (in other files) are this use case (specific to import string)