Fix python3 unicode issues

[sarg <sarg@github.com>]

Hi there. You've got nice workflow with managing rss feeds and I would
like to reproduce it. I've ran into minor issues with python3 and
unicode and was able to resolve them myself. Please take a look.

Note that this issue is copypasta from GitHub:

  https://github.com/Warbo/feed2maildir/pull/1

The relevant commit is a20179ba533c35cce3f94be2b979995a941aefc5 which
has now been merged.