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>

Add Gitpod integration mainly intended to make it easier for
contributors to work with RDFLib.

Other changes:

- Add a button for Google cloud shell integration, it is maybe not as
  nice as Gitpod but it provides a lot more free hours than Gitpod and it
  does work with the devcontainer based workflow even if the editor does
  not integrate with devcontainers directly.
- Removed additional python versions from the devcontainer dockerfile.
  This is mainly to make the devcontainer less unwieldy and quicker to
  build.
- Slim down the devcontainer Dockerfile by removing some unneeded
  packages, this will make it faster to build.

* Enable and baseline flakeheaven

This patch adds flakeheaven with a baseline, the baseline ignores all
existing errors and only reports on new errors.

The benefit with this approach is that we can get full flake8 on new
changes without having to first address the flake8 warnings with our
existing codebase.

Baselined errors can still be seen with:
- `flakeheaven lint --baseline /dev/null`
- `flake8`

This also adds a flakeheaven to GitHub Actions in a way which will
result in independent reporting.

Other changes:
- Simplified caching behaviour in GitHub Actions
- Removed flake8-black because it does not work well

  https://github.com/peterjc/flake8-black/issues/59
- Remove redundant/duplicated config for flake8

Co-authored-by: Christian Clauss <cclauss@me.com>

Added Taskfile with development tasks

This patch adds a Taskfile with various tasks that is useful for
RDFLib development. Details of the available tasks can be seen from
`task help` output and some more information is included in the RDFLib
developers guide.

For more information about Taskfiles see https://taskfile.dev/#/usage.

The GitHub action pipeline was also changed to now use tasks from the
Taskfile which simplifies the pipeline quite a bit.

This patch also adds development container which can be used to run
various development task and which can also be used with development
container enabled tools such as VSCode or GitHub codespaces.

This is all being done in preperation for moving coveralls reporting to
the GitHub actions workflow.

Other changes:
- Removed the `Makefile` and associated `test/Dockerfile` as it is now replaced by a `Taskfile.yml`.