Use `next()` instead of `six.advance_iterator`

Closes python-modernize/python-modernize#25
Merge pull request #36 from brettcannon/modern_next

Use `next()` instead of `six.advance_iterator`
README: say that we target Python 2.6+. refs #33

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Changelog entries since 0.3. refs #33, #36

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Changelog formatting.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Merge branch 'master' into unicode_callable