> v3 patch, based on feedback from the review here: http://bugs.python.org/review/14532/show

Looks good to me.
One last thing (sorry for not bringing this up earlier): I don't like
bikeshedding, but at least to me, `time_independent_equals` is a bit
too long to type, and sounds reductive (we don't want to specifically
avoid only timing attacks, but provide a way to compare digests
securely).
What do you (all) think of something shorter, like `secure_compare`,
`secure_equals`, or something along those lines?
Note that I'm not good at finding names, so if others are fine with
the current one, I won't object ;-)