use highest protocol version for pickling

[farwayer]

This is a very rare situation, when used old cache data from one environment to another with lower python version. I think it makes no sense to use the old ASCII protocol.

[ionrock]

@farwayer I actually looked at this an explicitly chose not to use the latest. The reason being is that I wanted to avoid the situation where you had a cache and the pickle format is updated (maybe a python version bump?) and you ruin your cache.

This might have been paranoia on my part. Do you mind letting me know your thoughts on upgrades causing cache corruption? I bet it is really easy to sway me on this ;)

Thanks for looking into it!

[farwayer]

I will try :)
All pickle protocols are forward compatible. That mean more new python version support old pickle protocols. So we only can get problem with downgrading python and than using old cache. This isn't very often situation imho. However thanks for describing this use case. I added checking it.

I also found encoding bug with unpickling py2 pickled cache in py3.

[ionrock]

Fantastic. Thank you @farwayer!