[entity] Add a cw_related_rqlst method returning the RQL select node of cw_related method
This is useful if one wants to modify the query before execution.
A bit a PEP8 style fix in tests along the way.
Related to #12306543.
Additional services
===================
In this chapter, we introduce services crossing the *web -
repository - administration* organisation of the first parts of the
CubicWeb book. Those services can be either proper services (like the
undo functionality) or mere *topical cross-sections* across CubicWeb.
.. toctree::
:maxdepth: 2
undo