[core] Adjust cw<3.21 compatibility
Most of cubibweb < 3.21 expects a ClientConnection as the main connection.
Related to #5731814
Closes #5878592
.. _login_module:
:mod:`pyramid_cubicweb.login`
-----------------------------
.. automodule:: pyramid_cubicweb.login
.. autofunction:: includeme
Views
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.. autofunction:: login_form
.. autofunction:: login_password_login
.. autofunction:: login_already_loggedin