python flow filter issue

[cyrush]

python 3.7.x

crash in python flow filter run time, when registering filters via flow.wrap_function()

demoed in t_python_flow_smoke.py test_basic_easy

no obvious culprit, here is a stack trace

    frame #15: 0x00000001000c4ba2 libpython3.7m.dylib`_PyFunction_FastCallDict + 450
    frame #16: 0x00000001000c60f6 libpython3.7m.dylib`_PyObject_Call_Prepend + 150
    frame #17: 0x00000001000c4938 libpython3.7m.dylib`_PyObject_FastCallDict + 360
    frame #18: 0x00000001000c62cf libpython3.7m.dylib`_PyObject_CallFunctionVa + 223
    frame #19: 0x00000001000c655b libpython3.7m.dylib`PyObject_CallMethod + 187
    frame #20: 0x0000000105fac767 flow_python.so`PyFlowFilter::declare_interface(this=0x00000001006267d0, iface=0x00007ffeefbfba40) at flow_python.cpp:412
    frame #21: 0x0000000105fadb68 flow_python.so`PyFlow_Workspace_register_filter_type((null)=0x0000000000000000, args=0x0000000105fa3f90) at flow_python.cpp:2318
    frame #22: 0x00000001000c5e0a libpython3.7m.dylib`_PyMethodDef_RawFastCallKeywords + 746
    frame #23: 0x00000001000c510c libpython3.7m.dylib`_PyCFunction_FastCallKeywords + 44
    frame #24: 0x000000010018eb59 libpython3.7m.dylib`call_function + 617
    frame #25: 0x00000001001873f0 libpython3.7m.dylib`_PyEval_EvalFrameDefault + 5088
    frame #26: 0x00000001000c55d2 libpython3.7m.dylib`function_code_fastcall + 210
    frame #27: 0x000000010018ebc6 libpython3.7m.dylib`call_function + 726