chore: mograte dev-dependencies to new poetry layout (#482)

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

Signed-off-by: Jan Kowalleck <[email protected]>

pyproject.toml

@@ -75,7 +75,7 @@ validation = ["jsonschema", "lxml"]
 json-validation = ["jsonschema"]
 xml-validation = ["lxml"]
 
-[tool.poetry.dev-dependencies]
+[tool.poetry.group.dev.dependencies]
 ddt = "1.6.0"
 coverage = "7.3.2"
 flake8 = { version="6.1.0", python=">=3.8.1" }