Migrate scripts from Python 2 to 3

When building the code in a Python3 only virtual environment, a build
failure occurred because of Python2 syntax. The 2to3 python script to
convert the contents of the gn-used file causing the issue, as well as
two other files.

Change-Id: I47855e96979ec42cd0fec0f01e99594c982f1738
diff --git a/tools/find_scan_roots.py b/tools/find_scan_roots.py
index b483bfb..f6bf892 100755
--- a/tools/find_scan_roots.py
+++ b/tools/find_scan_roots.py
@@ -43,7 +43,7 @@
     return n
 
   def __iter__(self):
-    for child in self.children.itervalues():
+    for child in self.children.values():
       yield self.name + '/' + child.name, child
       for p, ch in child:
         yield self.name + '/' + p, ch
@@ -56,7 +56,7 @@
 
     self.marked = True
 
-    for child in self.children.itervalues():
+    for child in self.children.values():
       child.Mark(labels)
 
     return True
@@ -89,7 +89,7 @@
   root = BuildTree()
   for fullpath, elem in root:
     if elem.Mark(args.labels):
-      print fullpath
+      print(fullpath)
 
 
 if __name__ == '__main__':