Change method _sort_key_for to static
Change method _sort_key_for to static

This method does not reference to any attribute of this class, so it's better
to change it to classstatic.

Change-Id: I3ea0ca83cb29ceae9afb20da0c9817a9b044c084
Signed-off-by: Lin Yang <984c62e29c173058275d217ee55de7058aa509b9@intel.com>

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