BUG: fix d and %dis in 3.7

[llllllllll]

As of Python 3.7, dis.dis has a similar recursive printing feature as d. This
causes us to print two copies of each code object in the tree.

This patch passes depth=0 to dis.dis for Python >= 3.7. I didn't just switch
to dis.dis because I think it produces worse labels, for example:

dis.dis:

In [3]: %dis [x for x in y]
  1           0 LOAD_CONST               0 (<code object <listcomp> at 0x7f2f262aa300, file "<dis>", line 1>)
              2 LOAD_CONST               1 ('<listcomp>')
              4 MAKE_FUNCTION            0
              6 LOAD_NAME                0 (y)
              8 GET_ITER
             10 CALL_FUNCTION            1
             12 RETURN_VALUE

Disassembly of <code object <listcomp> at 0x7f2f262aa300, file "<dis>", line 1>:
  1           0 BUILD_LIST               0
              2 LOAD_FAST                0 (.0)
        >>    4 FOR_ITER                 8 (to 14)
              6 STORE_FAST               1 (x)
              8 LOAD_FAST                1 (x)
             10 LIST_APPEND              2
             12 JUMP_ABSOLUTE            4
        >>   14 RETURN_VALUE

d:

In [2]: %dis [x for x in y]
<module>
--------
  1           0 LOAD_CONST               0 (<code object <listcomp> at 0x7f408047d270, file "<show>", line 1>)
              2 LOAD_CONST               1 ('<listcomp>')
              4 MAKE_FUNCTION            0
              6 LOAD_NAME                0 (y)
              8 GET_ITER
             10 CALL_FUNCTION            1
             12 POP_TOP
             14 LOAD_CONST               2 (None)
             16 RETURN_VALUE

<module>.<listcomp>
-------------------
  1           0 BUILD_LIST               0
              2 LOAD_FAST                0 (.0)
        >>    4 FOR_ITER                 8 (to 14)
              6 STORE_FAST               1 (x)
              8 LOAD_FAST                1 (x)
             10 LIST_APPEND              2
             12 JUMP_ABSOLUTE            4
        >>   14 RETURN_VALUE