Use setuptools entrypoint to register app

Advances and blocks AlekSIS-Core!395 (merged)

pyproject.toml

@@ -61,6 +61,10 @@ pytest-sugar = "^0.9.2"
 [tool.poetry.plugins."aleksis.app"]
 csv_import = "aleksis.apps.csv_import.apps:CSVImportConfig"
 
+[tool.black]
+line-length = 100
+exclude = "/migrations/"
+
 [build-system]
 requires = ["poetry>=1.0"]
 build-backend = "poetry.masonry.api"