fix isort add-imports config

Author: graingert

libmodernize/main.py

@@ -5,6 +5,7 @@
   |_|_|_\\___/\\__,_\\___|_| |_||_|_/__\\___|\
 """
 
+from __future__ import generator_stop
 
 import logging
 import optparse

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [tool.isort]
 profile = "black"
-add_imports="from __future__ import generator_stop"
+add_imports=["from __future__ import generator_stop"]
 
 [tool.pytest.ini_options]
 addopts = [