[core] Adjust cw<3.21 compatibility
Most of cubibweb < 3.21 expects a ClientConnection as the main connection.
Related to #5731814
Closes #5878592
.. _tools_module:
:mod:`pyramid_cubicweb.tools`
----------------------------
.. automodule:: pyramid_cubicweb.tools
.. autofunction:: includeme
.. autofunction:: clone_user
.. autofunction:: cnx_attach_entity
.. autofunction:: cached_build_user
.. autofunction:: clear_cache