Add backported subprocess.run to make sure it works in Python3.4

[raghur]

Thanks for the PR.. Couple of questions (I've not used subprocess module outside of ghost :))

1. In run, why not check existence of run and just call run, else call the backported code?
2. https://docs.python.org/3/library/subprocess.html#call-function-trio provides simpler alternatives to run. Given that we never need to pass stdin or read the output, does it make sense to use the simpler versions?

[balta2ar]

That totally makes sense! My knee-jerk reaction was to provide a backport of run, but it's actually way simpler to just use call, please take a look.

[raghur]

👍 Thanks - that looks good. So we can just go to subprocess.call.. Can you squash the commit? I'll merge it after that.

[balta2ar]

Done.