Update const.py

add forgotten commands:
# — delete a field.
! — insert a field (before the one specified).

basic test:
import tarantool
tnt = tarantool.connect()
tnt.insert(0, (0, 1), True)
r = tnt.update(0, 0, [(1, "!", "hello world")], True)
assert r == [('\x00\x00\x00\x00', 'hello world', '\x01\x00\x00\x00')], "FAIL"
r = tnt.update(0, 0, [(1, "#", "")], True)
assert r == [('\x00\x00\x00\x00', '\x01\x00\x00\x00')], "FAIL"