Use setuptools entrypoints to register AlekSIS app

pyproject.toml

@@ -62,6 +62,9 @@ flake8-isort = "^4.0.0"
 pytest-cov = "^2.8.1"
 pytest-sugar = "^0.9.2"
 
+[tool.poetry.plugins."aleksis.app"]
+untis = "aleksis.apps.untis.apps:UntisConfig"
+
 [build-system]
 requires = ["poetry>=0.12"]
 build-backend = "poetry.masonry.api"