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.
