Modify `python'` Variable in Package Using an Overlay

Currently the backintime-common package is broken due to a missing dependecy; the packaging python module. Its already been reported, but I’d like to use an overlay as a temporary fix.

So, I need to add packaging to the python' variable in common.nix. I’ve tried everything I can think of, but I’m missing some critical understanding, and can’t get anything to work.