- also consolidating other settings to pyproject.toml when possible

 - both envs fail when run - not clear that the failures are new since they seem unrelated to my changes

…"extra" html

… now

… str

…est poetry-core.

 - poetry-core is at 1.4.0; require that to avoid risk of encountering old bugs
 - "flake8" was causing problems with pip during validate jobs, in which pip
   tried to do this:

    py37: install_package_deps> python -I -m pip install black==22.12.0 'flake8>=4.0.1; or extra == "flake8"' 'flakeheaven<4.0.0,>=3.2.1; or extra == "flake8"' 'importlib_metadata<5.0.0,>=4.2.0; python_version >= "3.7" and python_version < "3.8"' 'isodate<0.7.0,>=0.6.1' 'isort<6.0.0,>=5.10.0' mypy==0.991 'networkx<3.0.0,>=2.6.2; (python_full_version > "3.7.0" and python_version < "3.8") and extra == "networkx"' 'networkx<3.0.0,>=2.8.8; (python_version >= "3.8" and python_version < "4.0") and extra == "networkx"' 'pep8-naming<0.14.0,>=0.13.2; or extra == "flake8"' 'pyparsing<4.0.0,>=3.0.9' 'pytest-cov<5.0.0,>=4.0.0' 'pytest<8.0.0,>=7.1.3'

    and complained about:

    pip._vendor.packaging.markers.InvalidMarker: Invalid marker: 'or extra == "flake8"', parse error at 'or extra'

…stead of requirements.txt files

…ependencies

 - exception: docker:prepare task is not yet updated

- `.github/workflows/validate.yaml`:
  - Cache the default `XDG_CACHE_HOME` instead of messing with
    `PIP_CACHE_DIR` and the variable. This simplifies caching quite a bit
    and also ensures caching works for poetry.
  - Install poetry for `extra-tasks`, so that poetry install works correctly.
  - Get an auth token in `extra-tasks` so we don't get hit by rate limiting.
- `docs/conf.py`:
  - Disable nit picking on pre-python 3.7 as this causes problems with older sphinx.
- `docs/developers.rst` and `docs/docs.rst`:
  - Incorporate poetry into instructions.
- `docker-compose.yml`:
  - Set `POETRY_VIRTUALENVS_IN_PROJECT=1` so that the `.venv` goes into a docker
    volume instead of going to emphemeral storage.
- `Dockerfile.devcontainer`
  - Don't install dependencies globally. With poetry, everyone should use
    poetry, and when using poetry the global python environment should not be
    used, so if people use the devcontainer correctly the global dependencies
    should have no effect.
- `poetry.lock`
  - Ran `poetry lock` after changes to `pyproject.toml`, most notable change is
    the stuff added to the dev Poetry dependency group is no longer optional.
- `pyproject.toml`:
  - Simplified the dependency constrains for `networkx` as this was causing
    problems with tox, as tox was running pip install with
    `'networkx<3.0.0,>=2.6.2; (python_version >= "3.7" and python_version <
    "3.8") and extra == "networkx"' 'networkx<3.0.0,>=2.8.8; (python_version >=
    "3.8" and python_version < "4.0") and extra == "networkx"'` and the `extra
    == "networkx"` here would never be true.
  - Added a `dev` poetry dependendcy group with everything that is needed for a
    sane development environment so that `poetry install` gets you a sane dev
    environment.
- `Taskfile.yml`:
  - Changed the commands so that everything works as before but now instead runs things with poetry.
  - Removed the legacy venv handling and replaced it with poetry equivalents.
- `tox.ini`:
  - Don't set nitpicky (`-n`) on the command line as it is being set
    conditionally on python version in `docs/conf.py` now.

poetry related changes

This is mainly so that poetry export is sane, but also because the extras should
not be polluted with things that peope who use the wheel don't want or use.

Other changes:

- GitHub Actions:
  - Use `abatilo/actions-poetry` instead of `snok/install-poetry` as snok does
    not work well on Windows.
  - Install poetry before running tox so tox can call poetry.
  - Fix caching on Windows and MacOS
- Add requirements files for poetry so its versions is managed by dependabot and
  so that the same version can be used in multiple contexts.
- Add requirements file for Sphinx version used in readthedocs.
- The problem with nitpicky was related to sphinx 4, not to python 3.7. So
  nitpicky is now disabled on Sphinx <= 5.
- `test/test_literal/test_xmlliterals.py`: Only run html5lib test when html5lib
  is present.
- Check poetry and poetry.lock from `.pre-commit-config.yaml`.
- Changed `.readthedocs.yaml` to use poetry.
- Added an `extra` for lxml.

This reverts commit fa1cc18.

Move dev dependencies from extras into poetry dep groups

This adds a tox environment and GHA test matrix entry for testing the
lowest allowed versions of dependencies.

test minimum versions and decrease the lower bounds for dependencies to earliest versions that pass

This reverts commit a4da002.

…o install poetry with all extras flag.

…mmands inside the devcontainer with docker volumes from the host

This should avoid interference between different containers and the host
environment.

Fix devcontainer stuff

change `Black` -> `isort` in isort config.