title: check_output doesn't works in python 3.6 while subprocess works fine
body: <p>i am trying to get the output of a command in my python program by using "check_output" method. but i'm getting this error: <br/></p>
  
  <pre><code>   out = check_output(command5 , shell=True)
  </code></pre>
  
  <p>File "/usr/lib64/python3.6/subprocess.py", line 336, in check_output
      **kwargs).stdout
    File "/usr/lib64/python3.6/subprocess.py", line 418, in run
      output=stdout, stderr=stderr)
  subprocess.CalledProcessError: Command 'oscap xccdf eval --profile xccdf_org.ssgproject.content_profile_rht-ccp --results-arf arf.xml /usr/share/xml/scap/ssg/content/ssg-centos7-ds.xml' returned non-zero exit status 2.</p>
  
  <p>this is the part of my program that is related:</p>
  
  <pre><code>command4 = "oscap xccdf eval --profile xccdf_org.ssgproject.content_profile_rht-ccp --results-arf arf.xml /usr/share/xml/scap/ssg/content/ssg-centos7-ds.xml"
  out = check_output(command4 , shell=True)
  </code></pre>
  
  <p>I am sure that the command is alright because I get the results when I write:</p>
  
  <pre><code>subprocess.call(command5,shell=True)
  </code></pre>
  
  <p>I am using python 3.6, and work in centos 7.</p>
  
  <p>any idea why the <code>check_output</code> can not get the result?</p>