fix flake8 ignore

Author: blink1073

setup.cfg

@@ -54,4 +54,4 @@ extend-ignore =
 per-file-ignores =
     # B011: Do not call assert False since python -O removes these calls
     # F841 local variable 'foo' is assigned to but never used
-    tests/*: B011, F841
+    jupyter_core/tests/*: B011, F841