Use binary file for devnull

[methane]

This is not a bug. But binary file is more efficient because TextIOWrapper is not used.

[hartwork]

Seems to work. I have tried:

importosimportsubprocesswithopen(os.devnull, 'wb') asdevnull:
argv= ['sh', '-c', 'echo stdout; echo stderr >&2']
subprocess.check_output(argv, stderr=devnull)

[methane]

We can use stderr=subprocess.DEVNULL isntead of opening it manually if we can drop Python 2 and 3.2 support.

[jdufresne]

We can use stderr=subprocess.DEVNULL isntead of opening it manually if we can drop Python 2 and 3.2 support.

I opened #281 to remove EOL Pythons, allowing for using DEVULL