Unexpected keyword argument 'bufsize' in subprocess.check_output()

On some python versions, chutney's call to subprocess.check_output() results in the following error:

Unexpected keyword argument 'bufsize'…

The check_output() documentation is unclear about which arguments are supported: it simply says that most arguments to Popen() are supported. (And bufsize is an argument to Popen().): https://docs.python.org/3.5/library/subprocess.html#subprocess.check_output https://docs.python.org/3.5/library/subprocess.html#subprocess.Popen

I think our best option here is to remove the bufsize argument, and accept the default buffered behaviour.

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