title: Added characters to string when exiting subprocces
body: <p>I am calling a subprocess and returning a string if there is an error.
  Code example:</p>
  
  <p>When calling the process:</p>
  
  <pre><code>def read_plan_with_break():
      comand = " python script.py "
      proc = subprocess.Popen(comand.split(), shell=False, stdout = subprocess.PIPE, stderr= subprocess.PIPE)
      if proc.wait() != 0:
          output, err = proc.communicate() 
          print (err)
          return "Error in subprocess"
      return True 
  </code></pre>
  
  <p>when exiting the subprocess:</p>
  
  <pre><code>def fatal_error():
    print("Some message", file=sys.stderr)
    exit(1)
  </code></pre>
  
  <p>My problem is that the <code>stderr</code> output is : <code>b'Some message\r\n'</code>
  I can erase the <code>\r\n</code> with <code>strip</code> but have no idea why there is a <code>b</code> at the beginning and the <code>'</code> at the start and the end.</p>
  
  <p>Does anyone know why this occurs?</p>
  
  <p>EDIT:
  I have tried <code>err.split()[2:-1]</code> to get rid of the <code>b'</code> but it cuts off the start of the <code>Some message</code></p>
  
  <p>If I get a down-vote, please explain so I can improve and make better questions in the future</p>