Explanation.

src/allmydata/web/directory.py

@@ -9,6 +9,7 @@ from __future__ import unicode_literals
 from future.utils import PY2
 if PY2:
     from future.builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, max, min  # noqa: F401
+    # Don't use Future's str so that we don't get leaks into bad byte formatting
     from past.builtins import unicode as str
 
 from urllib.parse import quote as url_quote