tox.ini: pin [testenv:docs] sphinx dependency to 1.8.5
Description
In sphinx 2.0.0 sphinx.apidoc has moved to sphinx.ext.apidoc, which causes a build import error. Pin the version to 1.8.5 to allow docs to be built until issues can be fixed.
'python3 -m sphinx.apidoc --force --separate --module-first --no-headings --no-toc -o source /builds/BuildStream/buildstream/doc/../buildstream _pb2.py /builds/BuildStream/buildstream/.tox/docs/bin/python3: No module named sphinx.apidoc'
See here for a failing job:
https://gitlab.com/BuildStream/buildstream/-/jobs/186621538