Ok, I did an experiment. I added "from __future__ import absolute_import" at the top of _multidict.c, and after a recompile the warning went away.
What changed was that the following line:
__pyx_t_1 = __Pyx_Import(__pyx_n_s_sys, 0, -1); if (unlikely(!__pyx_t_1)) __PYX_ERR(0, 5, __pyx_L1_error)
was replaced with:
__pyx_t_1 = __Pyx_Import(__pyx_n_s_sys, 0, 0); if (unlikely(!__pyx_t_1)) __PYX_ERR(0, 3, __pyx_L1_error)
Ignore the change from "3" to "5", which is the line number for the traceback. The relevant change is the "level" argument to __Pyx_Import() which went from -1 to 0.