Bug#975334: xeus-python ftbfs

[Gordon Ball]

Thanks for reporting.

This is a problem with pybind11-json-dev, which embeds the python
include path that it is built with, but does not currently declare a
dependency on the appropriate libpython3 version. (Not that declaring it
would be completely sufficient either, since as an arch:all it wouldn't
get binNMU'd anyway).

I'll make a new upload of pybind11-json-dev then giveback this package.