Get rid of set comprehension, as this doesn't exist in Python 2.6.
Move back to __import__ for 2.6
Don't rely on the ElementTree Comment factory being the tag attribute on Comments.

This is needed for xml.etree.cElementTree under 2.6 (and likely hence custom
installs of cElementTree 1.2), where the tag property is equal to
xml.etree.ElementTree.Comment (i.e., the pure Python version).

Similarly, don't rely on type(comment.tag), as this is just function!