Because without StrictUnicode both bytestrings (str from py2) and
unicode (unicode from py2 and str from py3) load into the same Go type
string so it becomes impossible to distinguish them from each other.
Re-saving data, thus, also generally introduces changes as e.g.
string loaded via bytestring will be saved as unicode when pickling with
protocol 3. Or a loaded unicode will be saved as bytestring with
pickling via protocol=2.

-> Switching to StrictUnicode mode solves all those problems.

Please see updated documentatin for zodbpickle.go and
https://github.com/kisielk/og-rek/commit/b28613c2 for more details about
StrictUnicode mode.

For ZODB/go this is change in behaviour exposed to outside. However
there is currently only one known ZODB/go user - WCFS in Wendelin.core -
and that user will be updated correspondingly as well.