Fix subprocess error exception call.
Merge pull request #454 from damianavila/fix_callesprocesserror

Fix subprocess error exception call.
Fixed missing datetime import.
Merge pull request #458 from damianavila/fix_dt

Fixed missing datetime import.
PyPy compatibility: esentially avoid pandas-related functionality and skipping some other test because NumPyPy does not pickle arrays yet.
Added default False to is_pypy.
Fixed the previous logic. It only checked for pandas-related stuff if pandas was installed, forgetting about the numpy possibilities.
Simplified logic using two main if flows, one for pandas objects and the other one for numpy objects.
Update to sync master.