Line length is there for a reason, it's to fit everything on the screen. Ignoring leading spaces/indentation or giving a 'fudge factor' doesn't help keep everything on the screen.

Ultimately there has to be a hard limit and it's silly to argue over special cases that should exceed it (because that's just another thing to bikeshed over in code reviews). Black sets a hard limit and enforces it--done, no more discussion.

IMHO it's a code smell to have a bunch of long lines of code that are all visually similar but vary in a tiny and easy to miss way. Here's another way to think about the code you wrote that boils it down to an even smaller and more focused intent:

    new = set(data.new_items)
    old = set(data.old_items)
    to_add = new - old
    to_remove = old - new

It's not exactly the same as what you wrote but you get the idea, and it can be made simpler if you're using set types to start with. It's kind of a nudge that if your intent is to do set-like operations like difference, intersection, etc. then you might want to use the right tools for the job instead of banging out more procedural code.