[Py2to3] Clean-up the use of keys() on dictionary objects.

To make the code more Python 3 compatible, I've made a few changes to how we handle keys() or iterkeys() calls on dictionaries. All functionality should remain the same.

 * Remove the use of .keys() or .iterkeys() when iterating through a dictionary.
 * Remove the use of .keys() when checking if key exists in dictionary.
 * Replace dict.keys() with list(dict) to obtain a list of dictionary keys. In Python 3 dict.keys() returns a dict_keys object, not a list.

msgfmt.py

@@ -60,9 +60,8 @@ def generate():
     """
     Return the generated output.
     """
-    keys = MESSAGES.keys()
     # the keys are sorted in the .mo file
-    keys.sort()
+    keys = sorted(MESSAGES)
     offsets = []
     ids = strs = ''
     for _id in keys: