pybind11 deprecation warnings

Created by: germasch

Sorry for posting so many issues, by the way. This one is a low priority one, but I am a bit annoyed with all the warnings I get from pybind11 using a deprecated (in python 3.7) API (warning: 'PyThread_set_key_value' is deprecated) when building adios2.

It looks like this has been fixed in upstream pybind11: https://github.com/pybind/pybind11/pull/1454, so it'd be great if pybind11 could be updated at some point.