The next three patches get to the point where (with mailgw.py hacked up)
demo.py successfully serves a login page under Python 3, although
actually logging in fails with more bytes/str issues.

65. Python 3 preparation: avoid basestring.

66. Python 3 preparation: update string translate method call in
cgi/accept_language.py.

The str translate method in Python 3 is sufficiently different from that
in Python 2 that different arguments are needed here.  Note that the
existing code "ascii = ''.join([chr(x) for x in range(256)])" is
redundant with both versions (in Python 2, None can be used instead as
the first translate argument from Python 2.6 onwards).

67. Python 3 preparation: use bytes in anti_csrf_nonce.