Libsemigroups-pybind11

This release increases the required version of ``libsemigroups`` to v2.7.1, which contains some bug fixes.

This is a minor release adding some new functionality from ``libsemigroups``:

* ukkonen: add support for Ukkonen + helpers by james-d-mitchell in https://github.com/libsemigroups/libsemigroups_pybind11/pull/132
* present: add further manip. funcs by james-d-mitchell in https://github.com/libsemigroups/libsemigroups_pybind11/pull/136

This is a minor release fixing some issues in the last release, namely:

* tests: add libsemigroups v2.6.2 special case by james-d-mitchell in https://github.com/libsemigroups/libsemigroups_pybind11/pull/133
* knuth-bendix: fix `helper to_latin1` again by james-d-mitchell in https://github.com/libsemigroups/libsemigroups_pybind11/pull/134

This is a minor release with some minor improvements:

* stephen: make `accepts/is_left_factor` call more robust by james-d-mitchell in https://github.com/libsemigroups/libsemigroups_pybind11/pull/129
* tests: add libsemigroups v2.7.0 special case by james-d-mitchell in https://github.com/libsemigroups/libsemigroups_pybind11/pull/130

**Full Changelog**: https://github.com/libsemigroups/libsemigroups_pybind11/compare/v0.9.0...v0.9.1

This is a minor release including two changes and some minor improvements to the doc:

* Improve ``ActionDigraph`` colours by MTWhyte in https://github.com/libsemigroups/libsemigroups_pybind11/pull/125
* Add support for ``ActionDigraph::cbegin/cend_pstilo`` by james-d-mitchell in https://github.com/libsemigroups/libsemigroups_pybind11/pull/127

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_semigroup``