The patch looks reasonable. I built on it with the following changes:
1. Added some extra test cases to cover Unicode format strings,
since the code was changed to handle these as well.
2. Changed test_builtin.py by
s/m[0].message.message/str(w[0].message)/, since
BaseException.message was deprecated in 2.6.
I also have the following general comments:
1. PEP 3101 explicitly defines the string conversion for
object.__format__. What is the rationale behind this? Should
we find out before making this change?
2. I don't think the comments in 'abstract.c' and 'typeobject.c'
explaining that the warning will eventually become an error are
needed. I think it would be better to open separate issues for
these migration steps as they can be tracked easier and will be
more visible.
3. test_unicode, test_str have cases that trigger the added
warning. Should they be altered now or when (if) this becomes
an error?