Allow comparison of TZ and non-TZ timestamps in sitemap plugin

Date comparisons now allow mixed TZ/non-TZ datetime objects. Previously, if an article, etc., had a date with a time zone but no modification time, a comparison between the date and datetime.min (which has no zone information) caused a crash.

This works well as a short-term fix, but it is preferable to fix the issue within pelican itself. Unzoned date information should be automatically given the time zone from the site configuration.