Switch from Poetry dev deps to dependency groups

Poetry first switched from dev dependencies [to dependency groups](python-poetry/poetry#4260) in
version
[1.2.0a2](https://github.com/python-poetry/poetry/blob/master/CHANGELOG.md#120a2---2021-08-01). The dev dependencies are currently deprecated and will be removed at some point, so this change switches to the new groups configuration.

For more information about dependency groups, see [the section in the
official documentation](https://python-poetry.org/docs/master/managing-dependencies#dependency-groups).

pyproject.toml

@@ -20,7 +20,7 @@ python = ">=3.7,<4"
 poetry = ">=1.2.0a1"
 python-dotenv = ">=0.10.0"
 
-[tool.poetry.dev-dependencies]
+[tool.poetry.group.dev.dependencies]
 pytest = "^6.2.3"
 
 [build-system]