contrib: remove one test from the python 3 whitelist 439b4d005b4a introduced a use of 'hg revert --no-backup' which is currently broken. Rather than try and do a fix on the (complicated) revert code during the freeze, let's just give up on this test under Python 3 until later. Once we do fix things, the test ratchet script will re-enable it.

ui: fix configbytes isinstance check to look for bytes and not str Fixes configbytes on Python 3.