Revert "Merge pull request #408 from damianavila/fix_shelve"

This reverts commit 95642581a9b30703e75056c2b1f1fc11bd174f3a, reversing
changes made to b1023b0e0b1865f70efe9bd2c04d746356f9081b.

I found some problems with py3 compatibility in the current implementation and in master **without** this PR merged, so we need to fix the previous errors first before trying to fix this one.