[2025.1] Pin setuptools in tox docs job

setuptools 82.0.0 removed pkg_resources, breaking stable/2025.1 jobs.
Use a temporary stable-only pin to setuptools==81.0.0.

Detail reasoning in [1].

[1] https://review.opendev.org/c/openstack/requirements/+/976227

Change-Id: If9a1842173e69145914f5d1ca6a4485e8910c258
Signed-off-by: Pierre Riteau <pierre@stackhpc.com>

Author: priteau

doc/requirements.txt

@@ -7,3 +7,6 @@ reno>=3.1.0 # Apache-2.0
 sphinx>=2.0.0,!=2.1.0 # BSD
 sphinxcontrib-svg2pdfconverter>=0.1.0 # BSD
 whereto>=0.3.0 # Apache-2.0
+# NOTE(priteau): pin setuptools
+# https://setuptools.pypa.io/en/stable/history.html#v82-0-0
+setuptools==81.0.0