fix(py3): Match wiki porting conventions

Address feedback from PR pointing out that my python-future imports aren't matching the
conventions from the wiki.

Trac: refs #3448

src/allmydata/scripts/common.py

@@ -5,7 +5,9 @@ import codecs
 from os.path import join
 
 # BBB: Python 2 compatibility
-from builtins import str
+from future.utils import PY2
+if PY2:
+    from future.builtins import str  # noqa: F401
 from six.moves.configparser import NoSectionError
 
 from twisted.python import usage

src/allmydata/scripts/stats_gatherer.py

@@ -3,7 +3,9 @@ from __future__ import print_function
 import os
 
 # BBB: Python 2 compatibility
-from builtins import str
+from future.utils import PY2
+if PY2:
+    from future.builtins import str  # noqa: F401
 
 from twisted.python import usage
 

src/allmydata/scripts/tahoe_check.py

@@ -4,7 +4,9 @@ import urllib
 import json
 
 # BBB: Python 2 compatibility
-from builtins import str
+from future.utils import PY2
+if PY2:
+    from future.builtins import str  # noqa: F401
 
 from twisted.protocols.basic import LineOnlyReceiver
 

src/allmydata/stats.py

@@ -7,7 +7,9 @@ import time
 from collections import deque
 
 # BBB: Python 2 compatibility
-from builtins import str
+from future.utils import PY2
+if PY2:
+    from future.builtins import str  # noqa: F401
 
 from twisted.internet import reactor
 from twisted.application import service

src/allmydata/test/check_load.py

@@ -38,7 +38,9 @@ import os, sys, httplib, binascii
 import urllib, json, random, time, urlparse
 
 # BBB: Python 2 compatibility
-from builtins import str
+from future.utils import PY2
+if PY2:
+    from future.builtins import str  # noqa: F401
 
 if sys.argv[1] == "--stats":
     statsfiles = sys.argv[2:]

src/allmydata/test/check_memory.py

@@ -3,7 +3,9 @@ from __future__ import print_function
 import os, shutil, sys, urllib, time, stat, urlparse
 
 # BBB: Python 2 compatibility
-from builtins import str
+from future.utils import PY2
+if PY2:
+    from future.builtins import str  # noqa: F401
 from six.moves import cStringIO as StringIO
 
 from twisted.internet import defer, reactor, protocol, error

src/allmydata/test/mutable/test_version.py

@@ -3,7 +3,9 @@ from __future__ import print_function
 import os
 
 # BBB: Python 2 compatibility
-from builtins import str
+from future.utils import PY2
+if PY2:
+    from future.builtins import str  # noqa: F401
 from six.moves import cStringIO as StringIO
 
 from twisted.internet import defer

src/allmydata/test/test_auth.py

@@ -1,5 +1,7 @@
 # BBB: Python 2 compatibility
-from builtins import str
+from future.utils import PY2
+if PY2:
+    from future.builtins import str  # noqa: F401
 
 from twisted.trial import unittest
 from twisted.python import filepath