setup: only write some autogenerated files if they change Without this change, setup.py always writes some files on every invocation. This prevents some builds from being a no-op when they should. And, since times can sneak into generated .pyc files, this prevents file content from being deterministic between builds. As part of the refactor, we treat file content as bytes. The only potential regression from this would be if some tool is looking at mtimes of the changed files to determine if further action should be taken. But I don't think anything critically important is keyed off the mtimes of these specific files. Differential Revision: https://phab.mercurial-scm.org/D1580