Use sorted() rather than .sort()

dict.keys() produces a dict_keys object in Python 3, which does
not support .sort(). Use sorted() which will give the same outcome.

Change-Id: If6b33db07a31995b4e44959209d08d8fb74ae339

manifest_xml.py

@@ -872,10 +872,8 @@ class XmlManifest(object):
     fromProjects = self.paths
     toProjects = manifest.paths
 
-    fromKeys = fromProjects.keys()
-    fromKeys.sort()
-    toKeys = toProjects.keys()
-    toKeys.sort()
+    fromKeys = sorted(fromProjects.keys())
+    toKeys = sorted(toProjects.keys())
 
     diff = {'added': [], 'removed': [], 'changed': [], 'unreachable': []}