[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.
.. _req_module:
:mod:`cubicweb.req`
===================
.. automodule:: cubicweb.req
.. autoexception:: FindEntityError
.. autoclass:: RequestSessionBase
:members: