libsemigroups/libsemigroups_pybind11: 0.8.0

This is a minor version that adds all the features from the namespace libsemigroups::presentation to the package, these are: add_zero_rules are_rules_sorted change_alphabet letter and character first_unused_letter greedy_reduce_length longest_rule, shortest_rule, longest_rule_length, and shortest_rule_length make_semigrou