locking: Use frozenset instead of utils.UniqueSequence

In this case “frozenset” is good enough as the result's order doesn't
matter--it is the input to “sorted” (“utils.UniqueSequence” preserves
the order). “frozenset” is ca. 25% faster for this use-case.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

lib/locking.py

@@ -1191,7 +1191,7 @@ class LockSet:
     # they will still be there after, but this makes it a lot faster should
     # just one of them be the already wrong. Using a sorted sequence to prevent
     # deadlocks.
-    for lname in sorted(utils.UniqueSequence(names)):
+    for lname in sorted(frozenset(names)):
       try:
         lock = self.__lockdict[lname] # raises KeyError if lock is not there
       except KeyError: