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I have just tested. BUILD_CONST_KEY_MAP doesn't used in it, because at this time -0. still is not a constant, but an expression (negation of 0.). With -0 it doesn't work too.
$ ./python -m dis
{0: 1, 2: 3}
 1 0 LOAD_CONST 0 (1)
 2 LOAD_CONST 1 (3)
 4 LOAD_CONST 2 ((0, 2))
 6 BUILD_CONST_KEY_MAP 2
 8 POP_TOP
 10 LOAD_CONST 3 (None)
 12 RETURN_VALUE
$ ./python -m dis
{-0: 1, 2: 3} 
 1 0 LOAD_CONST 5 (0)
 2 LOAD_CONST 1 (1)
 4 LOAD_CONST 2 (2)
 6 LOAD_CONST 3 (3)
 8 BUILD_MAP 2
 10 POP_TOP
 12 LOAD_CONST 4 (None)
 14 RETURN_VALUE