Attempt to get working in python2 and 3 at the same time

Finding https://github.com/concordusapps/wsgi-intercept from
@concordusapps inspired me to revisit this.

With their work, plus the stuff I've recently done for tiddlyweb
I had enough gumption to move things along.

I've chosen not to use six as the extent of differences is
quite small and I prefer having the changes be quite visible.
Add support for requests

I started from the work at https://github.com/concordusapps/wsgi-intercept
from @concordusapps but then decided to use the mixin style used
for httplib2.

That style may be useful for the urllib versions. Not as sure about
http_lib. Definitely further improvements that can be done here.

Further tidies coming shortly.