Libsemigroups-pybind11

This is a very minor release which contains no functional changes, or bugfixes, but exists to allow [zenodo.org](https://zenodo.org) to pick up the release and generate a doi for the package.

Update for version 2.3.2 of ``libsemigroups`` which contains some bugfixes.

This release is a minor update for version 2.3.1 of ``libsemigroups`` which contains some bugfixes in the ``Sims1`` class, the ``Presentation`` class and its helper functions.

This is a minor release with a couple of new features added:
* ``libsemigroups`` constants ``POSITIVE_INFINITY``, ``NEGATIVE_INFINITY``, and ``UNDEFINED`` are properly supported;
* ``libsemigroups::matrix_helper::pow`` is added as a method for ``__pow__`` for some types of matrices (those not defined over a runtime semiring);
and some minor improvements (the tests now use ``pytest`` exclusively).

This is a minor release adding support for the ``libsemigroups`` class ``Sims1`` for computing low index congruences, and some further minor changes to the ``Presentation`` class.

A very minor release to futureproof some tests against new versions of ``libsemigroups``.