Python3 emits index entries using bytes, but index.go was expecting only
bytestrings. As the result loading index produced by py3 was failing
e.g. as

    index_test.go:201: index load: testdata/py3_pickle3/1.fs.index: pickle @6: invalid oidPrefix: type ogórek.Bytes

-> Fix it by accepting both bytes and bytestrings inside the index.