Python: Context Manager

[andre-merzky]

This is related to #130: should we provide a context manager interface?:

    with jpsi.JobExecutor(url) as jex:

        job = jpsi.Job(jpsi.JobSpec("executable"='/bin/true'))
        jex.submit(job)
        ...

That would serve a similar purpose as the close() in #130, to free backend resources when jex goes out of scope. I assume that this will not be used in somewhat complex use cases where jex is unlikely to live in a simple scope as above, but it does make canonical examples simple and intuitive...

[hategan]

I think we talked about this during one of our meetings when a concern was voiced about (possibly unnecessary) API growth. In that spirit, I think there should be a rather high and clear need before anything should be added.

[andre-merzky]

Let's wait for #130 to get resolved - if that gets closed then there is no need for a ContextManager either...