gh-123340: Show string value of IS_OP oparg in dis

[sobolevn]

After:

» echo 'a is not b' | ./python.exe -m dis
  0           RESUME                   0

  1           LOAD_NAME                0 (a)
              LOAD_NAME                1 (b)
              IS_OP                    1 (is not)
              POP_TOP
              RETURN_CONST             0 (None)

and

» echo 'a is b' | ./python.exe -m dis    
  0           RESUME                   0

  1           LOAD_NAME                0 (a)
              LOAD_NAME                1 (b)
              IS_OP                    0 (is)
              POP_TOP
              RETURN_CONST             0 (None)

• Issue: dis: IS_OP should provide the name of the op #123340

[picnixz]

Small nitpick.

[sobolevn]

Thank you both for the reviews!