wcfs: zdata: Replace pycompat.Int64 with ogórek.AsInt64

Our "as int64" code was put into ogórek as generalized utility in
https://github.com/kisielk/og-rek/commit/010fbd2e.

/proposed-for-review-at !27 (comment 217042)

wcfs/internal/pycompat/pycompat.go

-// XXX dup from neo/go/zodb/internal/pickletools
-
-package pycompat
-
-import (
-	"math/big"
-)
-
-// Int64 tries to convert unpickled Python value to int64.
-//
-// (ogórek decodes python long as big.Int)
-//
-// XXX + support for float?
-func Int64(xv interface{}) (v int64, ok bool) {
-        switch v := xv.(type) {
-        case int64:
-                return v, true
-        case *big.Int:
-                if v.IsInt64() {
-                        return v.Int64(), true
-                }
-        }
-
-        return 0, false
-}

wcfs/internal/zdata/zblk.go

-// Copyright (C) 2018-2021  Nexedi SA and Contributors.
+// Copyright (C) 2018-2024  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -53,7 +53,6 @@ import (
 	"lab.nexedi.com/kirr/neo/go/zodb"
 	"lab.nexedi.com/kirr/neo/go/zodb/btree"
 
-	"lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/pycompat"
 	"lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xzodb"
 )
 
@@ -382,9 +381,9 @@ func (bf *zBigFileState) PySetState(pystate interface{}) (err error) {
 		return fmt.Errorf("expect [2](); got [%d]()", len(t))
 	}
 
-	blksize, ok := pycompat.Int64(t[0])
-	if !ok {
-		return fmt.Errorf("blksize: expect integer; got %s", xzodb.TypeOf(t[0]))
+	blksize, err := pickle.AsInt64(t[0])
+	if err != nil {
+		return fmt.Errorf("blksize: %s", err)
 	}
 	if blksize <= 0 {
 		return fmt.Errorf("blksize: must be > 0; got %d", blksize)