Added preference for stdlib's compare_digest to constant_time

Fixes #1210
Unfortunately, we can't do this :-( wheel tags are by major version, and hmac was introduced in 2.7.8, so we need all 2.7s to be compiled with support for this. I think for initial support we should just make it call `hmac.compare_digest` inside of `bytes_eq` if it's available, and change packaging stuff later.