Use triple double quotes instead of triple single quotes

src/utils/openmc/clean_xml.py

@@ -66,11 +66,11 @@ def sort_xml_elements(tree):
 
 
 def clean_xml_indentation(element, level=0):
-    '''
+    """
     copy and paste from http://effbot.org/zone/elementent-lib.htm#prettyprint
     it basically walks your tree and adds spaces and newlines so the tree is
     printed in a nice way
-    '''
+    """
 
     i = "\n" + level*"    "