Local development with PDM

With the move to meson-python we lost some of the convenience that Poetry brought us: managing venvs, local dependencies.

PDM allows us to add this back: virtual environment management, local dependencies (not part of extras_require). This tool also helps in setting up our environment for non-isolated builds, which is required to automatically recompile on changes in the C code.

There's one drawback: currently, when you install this project in your venv with:

pip install --no-build-isolation --config-settings=setup-args="-Dbuildtype=debug" -e '.[dev]'

You get debug builds, which make for easier debugging. However, for normal builds I do not want debug builds -- only for local development.

Note that it's still possible to install with pip, only the local dependencies, like pytest, are not installed automatically.