Merge pull request #3501 from cimarronm/text_xytext_bugfix

BUG : Bugfix for text.xytext property
Merge pull request #3477 from cimarronm/setupext_check_for_libpng

BLD : Better check for required dependency libpng
BUG : don't use super(self.__class__, self)

Simply remove super and explicitly call the correct base class.

closes #3493