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We were saving index data with bytestrings inside index records. This
works for py2 but on py3 it causes failure on unpickling because, by default,
*STRING opcodes are unpickled into unicode on py3:

    === RUN   TestIndexSaveToPy/py2_pickle1/py3
    UnicodeDecodeError: 'ascii' codec can't decode byte 0x8d in position 25: ordinal not in range(128)

    The above exception was the direct cause of the following exception:

    Traceback (most recent call last):
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/fs1/./py/indexcmp", line 43, in <module>
        main()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/fs1/./py/indexcmp", line 30, in main
        d1 = fsIndex.load(path1)
      File "/home/kirr/src/wendelin/z/ZODB/src/ZODB/fsIndex.py", line 138, in load
        v = unpickler.load()
    SystemError: <built-in method read of _io.BufferedReader object at 0x7fb631a86670> returned a result with an error set
        index_test.go:229: zodb/py read/compare index: exit status 1

-> Fix it by explicitly emitting index entries in binary form.

To be able to compare "index from py2 -> loaded into go -> saved by go ->
compared by py3" implement a bit of backward compatibility for  loading py2
index files on py3. Do this compatibility only for known-good py2 files, not
index produced by go code which is loaded by py3 without any compatibility
shims.