py3: finish porting iteritems() to pycompat and remove source transformer This commit finishes porting .iteritems() to pycompat.iteritems() for the mercurial package. The translation of .iteritems() to .items() was the last conversion performed by the source transformer. With the porting to pycompat complete, we no longer have a need for the source transformer. So the source transformer has been removed. Good riddance! The code base is now compatible with Python 2 and Python 3. For the record, as the person who introduced the source transformer, it brings me joy to delete it. It accomplished its goal to facilitate a port to Python 3 without overly burdening people on some painful low-level differences between Python 2 and 3. It is unfortunate we still have to wallpaper over many differences with the pycompat shim. But it is what it is. Differential Revision: https://phab.mercurial-scm.org/D7015