Add test script for py3 migration

[frisi]

After running python2 bin/zopeupdate --convert-py3 it would make sense to check if all database records can be unpickled w/o errors under python3

so run `zodbupdate --test-convert-py3``under python3 and list all oids, serials, classnames for pickles that lead to a UnicodeDecoreError.

@thefunny42 already did something similar that we can adapt and reuse in mdtools.relstorage: https://github.com/minddistrict/mdtools.relstorage/tree/master/src/mdtools/relstorage/search.py

[thefunny42]

This can leave in this repository, but that should be a different script I believe. As I repeat, zodbupdate is not just a Python3 migration tool.

[icemac]

This may be a good point to promote https://github.com/plone/zodbverify which may be really helpful here.