Brew is just to unstable

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Those changes are also compatible with Sphinx 2.x

The use of .. cpp:enumerator::  for a C enum is questionable,
but .. c:member:: no longer works in Sphinx 3 for the value of
a C enum, but .. c:enumerator:: is Sphinx 3 only
( https://www.sphinx-doc.org/en/master/usage/restructuredtext/domains.html#directive-c-enumerator )

Sphinx 3 is also more strict with the use of cross-reference
to C types. It rejects cross-references to things like
`` :c:type:`int` `` since this isn't a type we actually define.

Credits to @sebastic for his suggestion to try bionic and use aptitude
instead of apt !