Show diff in error msg when repr is long.
Fixes exception comparing bytes in py3

Result (failing/passing) should stay the same across pythons, but the
message on the AssertionError differs from 2 to 3.

So result is always tested, failmsg only for py3.
Fix byte test for py2.6

sys.version_info only became a namedtuple in 2.7, using positional
reference instead.
Cleanup comparing bytes test

Removing failmsg assertions, stdlib and repr can be trusted.
Leaving assertions that comparing bytes does function.
Merge branch 'longreprdiff'
run tests in Python 2.6, 2.7, 3.2, 3.4