TST/BUG: fix 2to3 import rewrite of import pickle

In pandas.io.pickle the following statement will not allow python to import
anything due to the following rewrite by 2to3

import pickle -> from . import pickle

This make the import try to import itself ad infinitum and thus fails.
TST: make tox block when running in parallel
Merge pull request #4063 from cpcloud/tox-pkl-import-fix

TST/BUG: fix 2to3 import rewrite of import pickle