Pickle protocol 3 allows to natively support all py bytes and string
types and is supported by zodbpickle/py on both py2 and py3 out of the
box. ZODB/py3 is using protocol=3 since 2013 and ZODB/py2 since 2018. In
other words nowdays protocol=3 is the default protocol with which
ZODB emits pickles, and it can be read practically everywhere.

The difference in between protocol=2 and protocol=3 is addition of
BINBYTES and SHORT_BINBYTES opcodes to represent bytes. Without those
opcodes bytes are encoded as

    `_codecs.encode(byt.decode('latin1'), 'latin1')`

which, when unpickled, results in bytes on py3 and str on py2.

Compared to using `BINBYTES byt` this form is much bigger in size, but
what is more important might turn bytes back into str when decoded and
reencoded on py2.

This form of bytes encoding is also not accepted by ZEO/py which rejects
it with

    2024-07-18T20:44:39 ERROR ZEO.asyncio.server Can't deserialize message
    Traceback (most recent call last):
      File "/home/kirr/src/wendelin/z/ZEO5/src/ZEO/asyncio/server.py", line 100, in message_received
        message_id, async_, name, args = self.decode(message)
      File "/home/kirr/src/wendelin/z/ZEO5/src/ZEO/asyncio/marshal.py", line 119, in pickle_server_decode
        return unpickler.load()  # msgid, flags, name, args
      File "/home/kirr/src/wendelin/z/ZEO5/src/ZEO/asyncio/marshal.py", line 176, in server_find_global
        raise ImportError("Module not allowed: %s" % (module,))
    ImportError: Module not allowed: _codecs

All in all using pickle protocol=3 is ok from both py2 and py3 point of
view, brings size optimization and correctness, and fixes ZEO and
probably other issues.

So let the pickles ZODB/go saves and other Go places emit be encoded with protocol=3 now as well.

For the reference, the

    // TODO 2 -> 3 since ZODB5 switched to it and uses zodbpickle.

is from 2019 added in a16c9e06 (go/zodb: Teach Persistent to serialize
itself).