Now we'll notice if a header forgets to include this magic phrase.
(I'm sympathetic to the argument that we should add a new linter test for each different check, instead of piggybacking every check into lint_headers.py (which we could rename to indicate that it checks nothing but include-ordering). However, I don't think we've reached the tipping point of complexity yet. :))
Nit: I think we rely on python3 exclusively now, so we could make this a Python3 f-string which I think is nicer. Thoughts? I hear the argument that this is consistent with the rest of the file (because it was mostly written when we only supported Python2).