title: Changing the default indentation of etree.tostring in lxml
body: <p>I have an XML document which I'm pretty-printing using <code>lxml.etree.tostring</code></p>
  
  <pre><code>print etree.tostring(doc, pretty_print=True)
  </code></pre>
  
  <p>The default level of indentation is 2 spaces, and I'd like to change this to 4 spaces.  There isn't any argument for this in the <code>tostring</code> function; is there a way to do this easily with lxml?</p>