go/zodb/zeo: Fix ZRPC/pickle binary and string encoding/decoding wrt py3

ZEO/go client was asserting "method" field to be bytestring. As the
result interoperation with ZEO/py3 server was failing as e.g.

    2024-07-18T20:41:14 INFO ZEO.asyncio.server received handshake 'Z5'
    2024/07/18 20:41:14 zlink @ - /tmp/zeo2281880331/1.fs.zeosock: serveRecv1: decode: .0: method: got string; expected str
        zeo_test.go:221: openzeo /tmp/zeo2281880331/1.fs.zeosock: zeo: open /tmp/zeo2281880331/1.fs.zeosock: zlink @ - /tmp/zeo2281880331/1.fs.zeosock: call register: zlink is closed

because ZEO/py3 sends "method" as unicode.

-> Fix it by using pickle.AsString when decoding "method" field.

Similarly ZEO/go client was sending strings as bytestr, which on py3 decode to
unicode ok only if they are ASCII. Adjust string encoding to be emitted
as unicode always which works wrt both ZEO/py3 and ZEO/py2.

--------

ZEO/go client was sending binary ZRPC arguments as bytestr as well,
which was resulting in the following exception on ZEO/py3 side:

    2024-07-18T20:42:31 ERROR ZEO.asyncio.server Can't deserialize message
    Traceback (most recent call last):
      File "/home/kirr/src/wendelin/z/ZEO/src/ZEO/asyncio/server.py", line 102, in message_received
        message_id, async_, name, args = self.decode(message)
      File "/home/kirr/src/wendelin/z/ZEO/src/ZEO/asyncio/marshal.py", line 122, in pickle_server_decode
        return unpickler.load()  # msgid, flags, name, args
    UnicodeDecodeError: 'ascii' codec can't decode byte 0x85 in position 1: ordinal not in range(128)
    2024/07/18 20:42:31 zlink @ - /tmp/zeo627526011/1.fs.zeosock: recvPkt: EOF
        xtesting.go:361: load 0285cbac70a3d733:0000000000000001: returned err unexpected:
            have: /tmp/zeo627526011/1.fs.zeosock: load 0285cbac70a3d733:0000000000000001: zlink @ - /tmp/zeo627526011/1.fs.zeosock: call loadBefore: zlink is closed
            want: nil

-> Fix it by using pickle.Bytes which should be encoded as bytes in the
   pickle stream.

This fix is ok but depends on the next patch to upgrade ZODB
serialization protocol to 3, because on pickle protocol 2 bytes are
serialized as `_codecs.encode(byt.decode('latin1'), 'latin1')` and
ZEO/py forbids import of _codecs:

    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

We will address this in the next patch.

go/zodb/storage/zeo/proto.go

@@ -151,10 +151,9 @@ func pktDecodeZ(pkb *pktBuf) (msg, error) {
 	// XXX check flags are in range?
 	m.flags = msgFlags(flags)
 
-	__, ok := tpkt[2].(pickle.ByteString)
-	m.method = string(__)
-	if !ok {
-		return m, derrf(".%d: method: got %T; expected str", m.msgid, tpkt[2])
+	m.method, err = pickle.AsString(tpkt[2])
+	if err != nil {
+		return m, derrf(".%d: method: %s", m.msgid, err)
 	}
 
 	m.arg = tpkt[3]
@@ -346,8 +345,8 @@ func (e encoding) xuint64Pack(v uint64) interface{} {
 		panic("bug")
 
 	case 'Z':
-		// pickle: -> str	XXX do we need to emit bytes instead of str?
-		return pickle.ByteString(mem.String(b[:]))
+		// pickle: -> zodbpickle.binary | bytes
+		return pickle.Bytes(mem.String(b[:]))
 
 	case 'M':
 		// msgpack: -> bin
@@ -407,9 +406,11 @@ func (e encoding) asString(xs interface{}) (string, bool) {
 
 	case 'Z':
 		// pickle: str
-		__, ok := xs.(pickle.ByteString)
-		s := string(__)
-		return s, ok
+		s, err := pickle.AsString(xs)
+		if err != nil {
+			return "", false
+		}
+		return s, true
 
 	case 'M':
 		// msgpack: bin(from py2) | str(from py3)