[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.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/html/index.html
Files: /usr/share/doc/cubicweb-documentation/html/*