Make tox test 3.3, drop 2.5
python-modernize --compat-unicode, works on py26/27

Also modifi all filter results in transport.py to be lists instead of
iterators, since there's a lot of len() calls that break.
Replace cStringIO with six.moves StringIO
Fix #103 problem with __slots__ in Python 3
Change UserDicts into MutableMappings
Weirdness with locals() between py2 and py3k
Modernize tests, fix broken tests for py2
Fix StringIO imports in tests
Do not run big sftp tests with --no-sftp
Stop stale threads in Python 3
Fix byte string problems with Python 3
Fix byte string problems in util
More byte string fixes
Test util and message more, create ord place-in
Replace superfluous use of bytearray with ord place-in
Replace chr() with six.int2byte()
Change remaining str()s to bytes()s
long() and string.find() are deprecated
Make Message encode strings with ascii
Make a dummy __delitem__ to satisfy MutableMapping