dev: use poetry's post 1.2 format for specifying dev dependencies

[muxator]

From https://python-poetry.org/docs/managing-dependencies/#dependency-groups:

A note about defining a dev dependencies group

The proper way to define a dev dependencies group since Poetry 1.2.0 is the following:

[tool.poetry.group.dev.dependencies]
pytest = "^6.0.0"
pytest-mock = "*"

This group notation is preferred since Poetry 1.2.0 and not usable in earlier versions.

[...]

Poetry will slowly transition away from the dev-dependencies notation which will soon be deprecated, so it’s advised to migrate your existing development dependencies to the new group notation.