使用以下内容:

from xml.dom import minidom

xml = """
<root>
 <x>text</x>
</root>""" 

dom = minidom.parseString(xml)
pi = dom.createProcessingInstruction('xml-stylesheet',
                                     'type="text/xsl" href="mystyle.xslt"')
root = dom.firstChild
dom.insertBefore(pi, root)
print dom.toprettyxml()

=>

<?xml version="1.0" ?>
<?xml-stylesheet type="text/xsl" href="mystyle.xslt"?>
<root>

   <x>
      text
   </x>

</root>