This change just wrecked Mercurial's release build process.

We've been building Mercurial release tarballs with a Makefile target wrapped around sdist for most of five years, and we've never had or wanted a MANIFEST.in file. We generate an exact, complete, and correct MANIFEST ourselves with:

 hg manifest > MANIFEST

This fix ignores that MANIFEST entirely and attempts to build a new one even though MANIFEST.in doesn't exist.

So not only does this change years of documented behavior in a way that very nearly caused us to ship a tarball missing over a thousand files, it means doing what we want to do with sdist (ship precisely the files listed in our version control manifest) is now significantly harder.

Unfortunately, this fix is now in a long-lived Python release which we support, which means we're stuck with it.