Replaced setuptools with poetry which provides simpler dependency management, dependency locking and virtual environment management.

This makes it easier for new developers to get a sane development environment (i.e. just `poetry install`) and also makes our CI pipeline more reproducable as now most development tools and libraries will only change through dependabot which will reduce the build failures on main and make breaking changes in dependencies more visible with dependabot PRs.

Other changes:
- moved isort, mypy and coverage config from setup.cfg to pyproject.toml
- Changes in the generated rdflib wheel:
  - removed wheel extras related to development (i.e. `tests`, `docs`, `dev`) as poetry dependency groups should be used instead.
  - Added an extra group `lxml` with a dependency on lxml.
  - Added version ranges to dependencies, and tests to ensure these version ranges are valid.
- Changes to tests:
  - Change pyparsing related tests so they work with older pyparsing.
  - Made the html5lib related tests conditional on html5lib existing.
- Runtime changes:
  - Use importlib.metadata to set `__version__`.
- Changed the devcontainer and related config to avoid contamination and interference with the host environment.

Co-authored-by: Iwan Aucamp <aucampia@gmail.com>
Co-authored-by: Edmond Chuc <edmond.chuc@outlook.com>

The diagram is based on the diagram created by
Graham Higgins (@gjhiggins) in https://github.com/RDFLib/rdflib/issues/1526

This shows the class heirarchy of various terms such as Identifier,
IdentifiedNode, URIRef, Literal, etc.

The diagram is in [plantuml](https://plantuml.com/class-diagram) and
compiled to svg by the
[kroki extension for sphinx](https://github.com/sphinx-contrib/kroki).

Diagrams can be rendered from the plantuml at https://kroki.io/.

Other changes:
 - Some updates to the "Writing RDFLib Documentation" page.