plist is generated to come with str in [0]. On py2 that is bytestring,
while on py3 it is unicode. However were trying to decode it as
bytestring only and so it was failing as

    --- FAIL: TestPersistentMapListLoad/py3_pickle3 (0.73s)
        persistent_x_test.go:82: plist[0]: got string  ; expect str

-> Fix it by using ogórek.AsString helper as suggested by
StrictUnicode=y mode documentation in https://github.com/kisielk/og-rek/commit/b28613c2.