Compilation warning

[martinRenou]

/home/travis/build/QuantStack/xeus-python/src/xutils.cpp: In function ‘zmq::message_t xpyt::pybytes_to_zmq_message(pybind11::bytes)’:
/home/travis/build/QuantStack/xeus-python/src/xutils.cpp:52:45: warning: conversion to ‘size_t {aka long unsigned int}’ from ‘Py_ssize_t {aka long int}’ may change the sign of the result [-Wsign-conversion]
         return zmq::message_t(buffer, length);