Fix Poetry deprecation warning

As suggested by `poetry lock`:

    The "poetry.dev-dependencies" section is deprecated and will be removed in a future version. Use "poetry.group.dev.dependencies" instead.

Author: amiryal

poetry.lock

@@ -10,7 +10,7 @@ python = "^3.6"
 feedparser = ">=6.0.5"
 html2text = ">=2020.1.16"
 
-[tool.poetry.dev-dependencies]
+[tool.poetry.group.dev.dependencies]
 update-copyright = ">=0.6.2"
 
 [build-system]