varLib.py. restore import of 'fontTools.misc.py23 import *'
varStore.py. Rename 'applyScalar' to 'interpolateFromDeltasAndScalars', and refactor common code with __git__item to a static method.
mutator.py. Update to reflect function name change to interpolateFromDeltasAndScalars. Changed some logic from one line to several lines for readability, and i to avoid flake8 warnings about line too long.