Use the new poetry way of configuring dev deps

https://python-poetry.org/docs/managing-dependencies/

{{cookiecutter.project_slug}}/pyproject.toml

@@ -34,7 +34,7 @@ packages = [
 [tool.poetry.dependencies]
 python = ">=3.8.1, <4.0"
 
-[tool.poetry.dev-dependencies]
+[tool.poetry.group.dev.dependencies]
 mkdocstrings = {version = ">=0.23", extras = ["python"]}
 mkdocs-material = "*"
 mypy = "*"