Migrate Poetry configuration from deprecated tool.poetry.dev-dependencies table

Originally Poetry supported two distinct dependency types, each defined separately in the `pyproject.toml` file:

* main: defined under the `tool.poetry.dependencies` table
* dev: defined under the `tool.poetry.dev-dependencies` table

This concept of dependency types was later made more flexible by allowing the user to define any number of arbitrary
dependency "groups", with "dev" simply being one of those groups. A new table naming scheme was created to support this
flexible dependency groups system.

For the sake of backwards compatibility, Poetry does still support use of the `tool.poetry.dev-dependencies` table.
However, it is deprecated and its use results in a warning.

The project's Poetry configuration is hereby migrated to the modern format.

Author: per1234

poetry.lock

@@ -6,7 +6,7 @@ package-mode = false
 [tool.poetry.dependencies]
 python = "~3.9"
 
-[tool.poetry.dev-dependencies]
+[tool.poetry.group.dev.dependencies]
 yamllint = "^1.37.1"
 codespell = "^2.4.1"