[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.
3.23 (UNRELEASED)
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Backwards-incompatible changes
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* Generative tests *à la logilab-common* are not supported anymore in
`CubicWebTC`. It is advised to use the subtests_ API (available on
`CubicWebTC` either from the standard library as of Python 3.4 or through
unittest2 package otherwise).
.. _subtests: https://docs.python.org/3/library/unittest.html#distinguishing-test-iterations-using-subtests