utils: Assert desired contract for normalize_paths()

This is a separate commit so it can be dropped during a rebase or revert
independently.

src/flake8/utils.py

@@ -168,6 +168,7 @@ def normalize_paths(paths, parent=os.curdir):
     :rtype:
         [str]
     """
+    assert isinstance(paths, list), paths  # nosec (for bandit)
     return [
         normalize_path(p, parent) for p in parse_comma_separated_list(paths)
     ]