get web.py ready for Python 3.0

[Aaron Swartz]

Running web.py under python2.6 -3 (the 3.0 warning mode) returns the following easy-to-fix errors:

web/utils.py:141: DeprecationWarning: the sets module is deprecated
  from sets import Set

(use the builtin set or frozenset instead)

web/webapi.py:20: DeprecationWarning: the cPickle module has been removed in Python 3.0
  import sys, cgi, Cookie, pprint, urlparse, urllib

(use pickle instead)

web/session.py:10: DeprecationWarning: the md5 module is deprecated; use hashlib instead
  import md5

web/wsgiserver/__init__.py:772: DeprecationWarning: callable() not supported in 3.x; use hasattr(o, '__call__')
  if callable(wsgi_app):

web/webapi.py:201: DeprecationWarning: comparing unequal types not supported in 3.x
  if expires < 0:

The problem here seems to be that expires defaults to "".