Module is not compatible with Python 2.7 anymore

We should be clear about that in documentation / `setup.py` / tests ... or fix it.

`plone.gallery` breaks on Python 2.7 because of (at least) :
 - `ModuleNotFoundError` in `gallery_shortcode.py` (which does not exist before Python 3)
 -  missing Unicode in `IRelatedPhotos` behavior

@MrTango would you drop compatibility or make it work on Python 2.7 again ?