Port unknown to Python 3

This is covered by test_dirnode/test_grid

src/allmydata/unknown.py

@@ -1,3 +1,13 @@
+"""Ported to Python 3.
+"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+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, str, max, min  # noqa: F401
 
 from zope.interface import implementer
 from twisted.internet import defer

src/allmydata/util/_python3.py

@@ -82,6 +82,7 @@ PORTED_MODULES = [
     "allmydata.storage.shares",
     "allmydata.test.no_network",
     "allmydata.test.mutable.util",
+    "allmydata.unknown",
     "allmydata.uri",
     "allmydata.util._python3",
     "allmydata.util.abbreviate",