repozo --verify

Could be used as a workaround for https://bugs.launchpad.net/zodb/+bug/906057
repozo --verify now correctly handles gzipped backups
Fix tests on Python 3
Refactor do_verify for readability

Extract size/checksum computation logic into separate functions for
gzipped and non-gzipped files.
Fix Py26 compat for 'with gzip.open'.
Merge pull request #14 from zopefoundation/repozo-verify

This adds 'repozo --verify' to check your backup integrity