message: Python 3.7 compatability
  The result of PyUnicode_AsUTF8() is now of type const char * rather of
  char *.
diff: @@ -916,7 +916,7 @@ bool pyopencv_to(PyObject* obj, String& value, const char* name)
  (void)name;
  if(!obj || obj == Py_None)
  return true;
  - char* str = PyString_AsString(obj);
  + const char* str = PyString_AsString(obj);
  if(!str)
  return false;
  value = String(str);
  