Migrate to PEP 735 dependency groups

[ncoghlan]

pdm 2.20.1+ supports PEP 735 dependency groups, which will allow migrating away from the PDM-specific development dependency declarations.

tox 4.22.0 also supports PEP 735 dependency groups: tox-dev/tox@568b90f

After migration, tox-pdm shouldn't be necessary anymore (since we're just using it to install specific PDM dependency groups in particular environments)

[ncoghlan]

#121 migrates to the standardised dependency group format (and takes advantage of group inclusion to make the purpose of the various entries in the "dev" group clearer)

tox-pdm remains as a dependency, as it turned out to be serving a second purpose: ensuring the tox environments respect the PDM lock file version pins.