Use PDM to manage dependencies and build site

[dairiki]

This is an alternative to #327, and is roughly equivalent to that PR except that it uses PDM rather than pipenv.

This uses PDM to manage python dependencies:

• Lektor version
• Published lektor plugins
• Local lektor plugin installation

It also uses PDM's scripts feature (very similar to npm scripts) to define common tasks.
E.g. once one has installed PDM and npm, one can just do:

# install python dependencies
pdm install

# build the site
pdm run build

This eliminates the need for the build/test instructions in tox.ini, and simplifies the build/deploy workflow.

TODO

Should probably add a pre-commit hook to ensure pdm.lock is up-to-date.