[pkg] Specify priority “optional” instead of the obsolete “extra” Quoting Lintian, “Since Debian Policy version 4.0.1, the priority extra has been deprecated.” So let's use “optional” instead.

.. _req_module:

:mod:`cubicweb.req`
===================

.. automodule:: cubicweb.req

    .. autoexception:: FindEntityError

    .. autoclass:: RequestSessionBase
        :members: