[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.

Release notes
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.. toctree::
    :maxdepth: 1

    3.22
    3.21
    3.20
    3.19
    3.18
    3.17
    3.16
    3.15
    3.14