bpo-37760: Mark all generated Unicode data headers as generated. (GH-15171)

This causes them to be collapsed by default in diffs shown on GitHub.

https://bugs.python.org/issue37760

Automerge-Triggered-By: @benjaminp
(cherry picked from commit 4e3dfcc)

Co-authored-by: Greg Price <[email protected]>

Author: miss-islington