Fix deprecation warning

This is the new to do it using dependency groups.
https://python-poetry.org/docs/1.8/managing-dependencies/

Author: vanillajonathan

pyproject.toml

@@ -7,7 +7,7 @@ authors = ["rs <[email protected]>"]
 [tool.poetry.dependencies]
 python = "^3.6.2"
 
-[tool.poetry.dev-dependencies]
+[tool.poetry.group.dev.dependencies]
 black = "^22.6.0"
 mypy = "^0.971"
 flake8 = "^5.0.4"