> Do you know why 255 became more common?

Because the line number is now correctly set for every bytecode instruction.

Compare the output in msg345108 for 3.8 with the corresponding output in 3.7:

  1           0 BUILD_LIST               0
              2 LOAD_FAST                0 (.0)
        >>    4 FOR_ITER                12 (to 18)

  2           6 STORE_FAST               1 (x)
              8 LOAD_FAST                1 (x)
             10 POP_JUMP_IF_FALSE        4
             12 LOAD_FAST                1 (x)
             14 LIST_APPEND              2
             16 JUMP_ABSOLUTE            4
        >>   18 RETURN_VALUE