Draft: Drop python2 support and bump the pybind11 version
Drop python2 support and bump the pybind11 version. The reasons are:
- Python 2 is obsolete and does not work with the newer pybind11 versions.
- Python 2 requires more extensions for the cmake setup.
The reasons to wait:
- Python2 is the default for CentOS7, which will be retired only in 1 year. So the milestone for this MR is either 3.3.0, or even some further versions if 3.3.0 will come before the retirement of CentOS7.