Migrate to Poetry v2 and project-managed Poetry plugins

[pronovic]

Poetry v2.0.0 was released on Jan 5. This is a major version, with some interface changes both in the Poetry command line and in the format of pyproject.toml. It also supports a feature I've been wanting for a long time, project plugins managed in pyproject.toml. This will let me use poetry-dynamic-versioning as a project plugin.

Most of the work here was in rewriting pyproject.toml to follow the new standard from PEP-621. After doing this, I confirmed that the generated artifacts in dist (created via run build) had only minor, explainable differences. This gave me confidence that the old and new versions of pyproject.toml were equivalent.

Since I was rewriting pyproject.toml anyway, I took the opportunity to move configuration for pytest, mypy, and coverage out of individual files like .mypy.ini and into pyproject.toml instead.

The poetry install --sync command is deprecated, so I updated commands/virtualenv.sh in run-script-framework to use poetry sync instead. I also made some other minor adjustments to the run script framework to ensure I got the intended behavior from some Poetry commands.

Now that project plugins are available, I was able to simplify the shared GitHub actions and workflows, which no longer need to be aware of plugins at all (see setup-poetry and gha-shared-workflows).

I also made changes in the cookiecutter-pypi template based on the work prototyped here.

[pronovic]

I opened poetry-dynamic-versioning issue #203 to report a minor formatting issue in pyproject.toml -- the version line always gets moved to the bottom of the [tool.poetry] stanza.

[pronovic]

I opened poetry issue #9975, because I noticed that Poetry now removes installed extras when adding a new dependency, which surprised me.