[entity/url publishing] fetch_rqlst should use 'is_instance_of' instead of 'is'
We usually want to get any subclasses as well (think Training subclass of
Workcase). Closes #3210365.
usr/lib/PY_VERSION/*-packages/cubicweb/entities/
usr/lib/PY_VERSION/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/PY_VERSION/*-packages/cubicweb/*.py