6950.bugfix

Increased the upper boundary of the :doc:`multidict:index` dependency
to allow for the version 6 -- by :user:`hugovk`.

It used to be limited below version 7 in :doc:`aiohttp <index>` v3.8.1 but
was lowered in v3.8.2 via :pr:`6550` and never brought back, causing
problems with dependency pins when upgrading. :doc:`aiohttp <index>` v3.8.3
fixes that by recovering the original boundary of ``< 7``.