Changed the HTTP connectivity from urllib to httlib

This enables persistent http connections (Keep-Alive) and is hence
slightly faster. Especially if you are behind a chain of web proxies and
update a lot of cells.
Removed the close() for HttpSessions.

The method was broken, not used & not tested. The sessions should be
closed when the process terminates anyways. If you need a close, because
you are running this inside of some container, it is easy enough to
iterate through the connections and close them all.
Fixed conflicts with @einvalentin's pull request
Returned to Python 3 support