In Python 3, cgi.FieldStorage needs to know about encodings like
shiftjis in order to decode it properly (in Python 2 it's simply not
decoded at all).

Fixes tests.test_wsgiwrappers.test_wsgirequest_charset

Add HTTP exception for new code 429 "Too Many Requests"

HTTP body must be bytes. Don't check if __iter__() method to check if content
is a list or tuple, because bytes and str now have this method on Python 3.
Instead, check explicitly for bytes and str types using the six module.

Port wsgiwrappers test to Python 3, fix bytes/unicode issues:

* HTTP body must be bytes
* Filenames are native strings, request ensures that filenames are unicode

Use "try/except ImportError" to try Python 2 and Python 3 names.

http://www.ietf.org/id/draft-nottingham-http-new-status-03.txt