XMLExportImport: don't support bytes for now

XMLExportImport encode strings in base64 when they can not safely be
represented in XML (including for exemple "\n"), so we can not use
this information to decide if this a BINBYTES or a BINSTRING pickle
opcode.

For now, it's still unclear how bytes and str should be handled, so
while using python2 it's better to keep using str everywhere and not
introduce zodbpickle.binary that looks like str on python2 but will
become bytes on python3 and have consistent data everywhere (ie.
everything str)

BINBYTES is not fully removed from the patch, because we'll want to
use it later, but it is in "if" that is always false.