> That's "should", not "must".
I read that “should” as a polite “must”.

> Also, I thought people did undocumented things with distutils, and we
> had to support these undocumented uses?
People rely on undocumented features and sometimes on bugs.  Thus, we cannot refactor or otherwise clean up internals.  Here, you did change internals.

> I don't see how adding "expected failures" solves anything. [...].
If a buildbot is red for a week because of me and another developer commits something that creates another test failure, they won’t see that a buildbot has turned red because it already was.  It’s just a temporary edition to avoid polluting the buildbots output.

> A "commit adding problem" should be fixed or reverted
The point is that fixing it may take tome.  Reverting is fine by me.