Modernise the build system

[Joseph-Edwards]

This PR modernises the build system, and makes the package adhere some Python good practices. Specifically, this PR does the following:

• Move as much project metadata out of setup.py into pyproject.toml as possible (see the Python Packaging Guide and PEP 518).
• Move some of the tools used for building libsemigroups_pybind11 into their own module to prevent circular includes.
• Refactor the code code that finds include directories and library directories.
• Use setuptools-scm to use git information to define the version of the module (see setuptools-scm).
• Consolidate and update the various environment.yml and requirements.txt files.
• Move libsemigroups_pybind11/ into src/ (see src layout vs flat layout).
• Update the list of authors and contributors.

[Joseph-Edwards]

@james-d-mitchell, this feels like a good point to specify authors and maintainers in the pyproject.toml. Presently, I've just moved them over from the old setup.py list, but we should probably update that. Are you happy for me to leave this decision to you?

[james-d-mitchell]

I have some questions, which we can discuss next week, but this looks really good

[Joseph-Edwards]

This has now been rebased on top of the latest v1 changes. Other than updating the list of authors, which could be deferred to a different PR, I think this is good to go.

[james-d-mitchell]

Looks good and happy to merge apart from the one (possible?) issue in the make-lint script.