Pickle protocol 5 adds out-of-band data and BYTEARRAY8 opcode (see [1]
and [2]). We add support for BYTEARRAY8 here. Handling out-of-band data
would require more setup from users - on both decoding and encoding
side, and it is likely that currently no practical use-case exists to
work with pickles with out-of-band data on Go side. This way we
behave as if no out-of-band data was provided when seeing all protocol 5
opcodes besides BYTEARRAY8.

Hopefully fixes: https://github.com/kisielk/og-rek/issues/62

[1] https://www.python.org/dev/peps/pep-0574
[2] https://github.com/python/cpython/commit/91f4380cedba

/reviewed-by @kisielk
/reviewed-on https://github.com/kisielk/og-rek/pull/63