switch to newer pybind11 version (2.6.1 from Nov 2020)

dune/python/pybind11/_version.py

dune/python/pybind11/attr.h

dune/python/pybind11/buffer_info.h

dune/python/pybind11/cast.h

dune/python/pybind11/chrono.h

dune/python/pybind11/complex.h

dune/python/pybind11/detail/class.h

dune/python/pybind11/detail/common.h

dune/python/pybind11/detail/descr.h

dune/python/pybind11/detail/init.h

dune/python/pybind11/detail/internals.h

dune/python/pybind11/detail/typeid.h

dune/python/pybind11/eigen.h

dune/python/pybind11/embed.h

dune/python/pybind11/eval.h

dune/python/pybind11/extensions.h

dune/python/pybind11/functional.h

dune/python/pybind11/iostream.h

dune/python/pybind11/numpy.h

dune/python/pybind11/operators.h