tests: avoid using a list comprehension to fill a list with fixed values

Flagged by PyCharm as an unused assignment for the variable in the list.

Differential Revision: https://phab.mercurial-scm.org/D7763

tests/test-cbor.py

@@ -1230,7 +1230,7 b' class BufferingDecoderTests(TestCase):'
             True,
             False,
             None,
-            [None for i in range(128)],
+            [None] * 128,
         ]
 
         encoded = b''.join(cborutil.streamencode(source))