Follow the pybind11 updating guide to migrate to pybind11 v3 #302
Description
On 10th July, version 3.0.0 of pybind11 was released. Fortunately, it looks like libsemigroups_pybind11 has not been impacted by any of the backwards-incompatible changes that have been made. Nonetheless, I think it would be sensible to follow the advice set out in the upgrade guide.