wcfs: zdata: Fix loading ZBlk data saved on py3

Previously TestZBlk was xfailing as e.g.

        --- FAIL: TestZBlk/py3_pickle3 (0.00s)
    panic: ZBlk0(0000000000000002): loadBlkData: wendelin.bigfile.file_zodb.ZBlk0(0000000000000002): activate: pysetstate: expect str; got ogórek.Bytes [recovered]
            panic: ZBlk0(0000000000000002): loadBlkData: wendelin.bigfile.file_zodb.ZBlk0(0000000000000002): activate: pysetstate: expect str; got ogórek.Bytes

because on py3 ZBlk data is saved as bytes while zblk.go code was trying
to decode it as bytestring (str from py2).

-> Fix that by adjusting ZBlk/go to follow py3 model with ZBlk data being
   considered to be bytes but also accepting bytestr from py2 for that.

Now on go side loading ZBlk works for all py2_pickle{1,2,3} and
py3_pickle3 data.

wcfs/internal/zdata/zblk.go

@@ -29,7 +29,7 @@ package zdata
 //	.blktab		LOBtree{}  blk -> ZBlk*(blkdata)
 //
 // ZBlk0 (aliased as ZBlk)
-//	str with trailing '\0' removed.
+//	bytes|bytestr with trailing '\0' removed.
 //
 // ZBlk1
 //	.chunktab	IOBtree{}  offset -> ZData(chunk)
@@ -79,7 +79,7 @@ type ZBlk0 struct {
 	zodb.Persistent
 
 	// NOTE py source uses bytes(buf) but on python2 it still results in str
-	blkdata pickle.ByteString
+	blkdata pickle.Bytes
 }
 
 type zBlk0State ZBlk0 // hide state methods from public API
@@ -96,9 +96,9 @@ func (zb *zBlk0State) PyGetState() interface{} {
 
 // PySetState implements zodb.PyStateful.
 func (zb *zBlk0State) PySetState(pystate interface{}) error {
-	blkdata, ok := pystate.(pickle.ByteString)
-	if !ok {
-		return fmt.Errorf("expect str; got %s", xzodb.TypeOf(pystate))
+	blkdata, err := pickle.AsBytes(pystate)
+	if err != nil {
+		return err
 	}
 
 	zb.blkdata = blkdata
@@ -125,7 +125,7 @@ type ZData struct {
 	zodb.Persistent
 
 	// NOTE py source uses bytes(buf) but on python2 it still results in str
-	data pickle.ByteString
+	data pickle.Bytes
 }
 
 type zDataState ZData // hide state methods from public API
@@ -142,9 +142,9 @@ func (zd *zDataState) PyGetState() interface{} {
 
 // PySetState implements zodb.PyStateful.
 func (zd *zDataState) PySetState(pystate interface{}) error {
-	data, ok := pystate.(pickle.ByteString)
-	if !ok {
-		return fmt.Errorf("expect str; got %s", xzodb.TypeOf(pystate))
+	data, err := pickle.AsBytes(pystate)
+	if err != nil {
+		return err
 	}
 
 	zd.data = data

wcfs/internal/zdata/zblk_test.go

@@ -67,9 +67,6 @@ func TestZBlk(t *testing.T) {
 	for _, zkind := range zkindv {
 		z := zblkTestDataRegistry[zkind]
 		t.Run(zkind, func(t *testing.T) {
-			if zkind == "py3_pickle3" {
-				t.Skip("xfail")
-			}
 			_TestZBlk(t, z)
 		})
 	}