Run python-modernize on the Python files under common/lib/capa in the edx-platform repository, to make them more compatible with Python 3.x without breaking Python 2.7 support:
python-modernize -w common/lib/capa
Make sure the changes look reasonable and submit them as a pull request; mention this ticket in the description.
Ask for tests to be triggered if they don't start automatically.
Fix any test failures caused by the changes.