docs/conf.py: Remove usage of distutils

docs/conf.py: Remove usage of distutils

The macOS jobs in our CI recently started failing, complaining that
the distutils module is not available anymore. And indeed, according to
https://peps.python.org/pep-0632/ it's been deprecated since a while
and now likely got removed in recent Python versions.

Fortunately, we only use it for a version check via LooseVersion here
which we don't really need anymore - according to Repology.org, these
are the versions of sphinx-rtd-theme that are currently used by the
various distros:

 centos_stream_8: 0.3.1
 centos_stream_9: 0.5.1
 fedora_38: 1.1.1
 fedora_39: 1.2.2
 freebsd: 1.0.0
 haikuports_master: 1.2.1
 openbsd: 1.2.2
 opensuse_leap_15_5: 0.5.1
 pkgsrc_current: 2.0.0
 debian_11: 0.5.1
 debian_12: 1.2.0
 ubuntu_20_04: 0.4.3
 ubuntu_22_04: 1.0.0
 ubuntu_24_04: 2.0.0

So except for CentOS 8, all distros are using a newer version of
sphinx-rtd-theme, and for CentOS 8 we don't support compiling with
the Sphinx of the distro anymore anyway, since it's based on the
Python 3.6 interpreter there. For compiling on CentOS 8, you have
to use the alternative Python 3.8 interpreter which comes without
Sphinx, so that needs the Sphinx installed via pip in the venv
instead, and that is using a newer version, too, according to our
pythondeps.toml file.

Thus we can simply drop the version check now to get rid of the
distutils dependency here.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Message-id: 20240304130403.129543-1-thuth@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>