After 2c359fc1 (decoder: Remember protocol version as last seen in a
PROTO opcode) BenchmarkDecode started to fail with

    --- FAIL: BenchmarkDecode
        ogorek_test.go:977: invalid pickle version

That happens because some input pickles now come with `PROTO 0xff`
prefix and version 0xff is indeed invalid.

-> Fix it by adjusting that prefix to be `PROTO 3` during benchmark as 3
is the most-widely used protocol version used by both py3 and py2 (via
zodbpickle).