fix(cmd): checking process.DEVNUL were needlessly opening `os.devnull`
fix(cmd): checking process.DEVNUL were needlessly opening `os.devnull`

Fixes resource-leak warning on Windows Puython-3.5.3+:

D:\python-3.5.2.amd64\lib\site-packages\git\cmd.py:583: ResourceWarning:
unclosed file <_io.BufferedWriter name='nul'>
  else getattr(subprocess, 'DEVNULL', open(os.devnull, 'wb')))
gitpython-developers/gitpython,gitpython-developers/GitPython,jeblair/GitPython,jeblair/GitPython,breavo/GitPython,breavo/GitPython,gitpython-developers/GitPython,gitpython-developers/gitpython