math/cadabra2: unbreak with libc++ 7

In file included from core/PythonCdb.cc:25:
In file included from libs/pybind11/include/pybind11/stl.h:34:
/usr/include/c++/v1/experimental/optional:11:2: error: "<experimental/optional> has been removed. Use <optional> instead."

PR: 230355
Obtained from: upstream (pybind11)