Added casting to string of slurm_stderr and jobinfo_stdout

[pietromarchesi]

For compatibility with Python 3, where subprocess is returning bytes instead of string. This fix was discussed on this issue. I have tested this on a Slurm cluster with both a Python 3.6 environment and a Python 2.7 environment, so the casting with str seems to work. There is a possibility to specify the encoding, using .decode('utf-8') or similar. I don't know when/if this would be necessary.

[samuell]

Cool, thanks @pietromarchesi ! This looks to be a clear improvement over the current situation, so merging.

I realize its due time to set up continuous integration, to be safer about merging things in the future. Hope to look at that in the coming week or weeks (a bit entrenched in deep a scipipe refactoring right now ;) ... so after that).