mysterious docs building failures in python3.8 builds - AttributeError on line_length_limit

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over in #1349, we have some warnings and failures that are unconnected to the changes there -

WARNING: `mathjax_config['tex2jax']` is set, but `myst_update_mathjax = True`, and so this will be overridden. Set `myst_update_mathjax = False` if you wish to use your own config

and

  Exception occurred:
    File "/Users/runner/work/sourmash/sourmash/.tox/docs/lib/python3.8/site-packages/docutils/parsers/rst/__init__.py", line 196, in parse
      if len(line) > self.document.settings.line_length_limit:
  AttributeError: 'Values' object has no attribute 'line_length_limit'
  The full traceback has been saved in /var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/sphinx-err-_txu1fpb.log, if you want to report the issue to the developers.
  Please also report this if it was a user error, so that a better error message can be provided next time.
  A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
  ERROR: InvocationError for command /Users/runner/work/sourmash/sourmash/.tox/docs/bin/sphinx-build -d /Users/runner/work/sourmash/sourmash/.tox/docs_doctree doc /Users/runner/work/sourmash/sourmash/.tox/docs_out --color -bhtml (exited with code 2)

some background info:

.package recreate: /Users/runner/work/sourmash/sourmash/.tox/.package
.package installdeps: setuptools >= 48, setuptools_scm[toml] >= 4, <6, setuptools_scm_git_archive, milksnake, wheel >= 0.29.0
docs create: /Users/runner/work/sourmash/sourmash/.tox/docs
docs installdeps: pip>=19.3.1
docs inst: /Users/runner/work/sourmash/sourmash/.tox/.tmp/package/1/sourmash-4.0.1.dev19+g5c450d7.tar.gz
docs installed: /Users/runner/work/sourmash/sourmash/.tox/docs/lib/python3.8/site-packages/pip/_vendor/packaging/version.py:127: DeprecationWarning: Creating a LegacyVersion has been deprecated and will be removed in the next major release,  warnings.warn(,/Users/runner/work/sourmash/sourmash/.tox/docs/lib/python3.8/site-packages/pip/_vendor/packaging/version.py:127: DeprecationWarning: Creating a LegacyVersion has been deprecated and will be removed in the next major release,  warnings.warn(,alabaster==0.7.12,appnope==0.1.2,async-generator==1.10,attrs==20.3.0,Babel==2.9.0,backcall==0.2.0,bleach==3.3.0,bz2file==0.98,cachetools==4.2.1,certifi==2020.12.5,cffi==1.14.5,chardet==4.0.0,cycler==0.10.0,decorator==5.0.4,defusedxml==0.7.1,deprecation==2.1.0,docutils==0.17,entrypoints==0.3,idna==2.10,imagesize==1.2.0,ipython==7.22.0,ipython-genutils==0.2.0,jedi==0.18.0,Jinja2==2.11.3,jsonschema==3.2.0,jupyter-client==6.1.12,jupyter-core==4.7.1,jupyterlab-pygments==0.1.2,kiwisolver==1.3.1,markdown-it-py==0.6.2,MarkupSafe==1.1.1,matplotlib==3.4.1,mdit-py-plugins==0.2.6,mistune==0.8.4,myst-parser==0.13.5,nbclient==0.5.3,nbconvert==6.0.7,nbformat==5.1.3,nbsphinx==0.8.2,nest-asyncio==1.5.1,numpy==1.20.2,packaging==20.9,pandocfilters==1.4.3,parso==0.8.2,pexpect==4.8.0,pickleshare==0.7.5,Pillow==8.2.0,pockets==0.9.1,prompt-toolkit==3.0.18,ptyprocess==0.7.0,pycparser==2.20,Pygments==2.8.1,pyparsing==2.4.7,pyrsistent==0.17.3,python-dateutil==2.8.1,pytz==2021.1,PyYAML==5.4.1,pyzmq==22.0.3,requests==2.25.1,scipy==1.6.2,screed==1.0.5,six==1.15.0,snowballstemmer==2.1.0,sourmash @ file:///Users/runner/work/sourmash/sourmash/.tox/.tmp/package/1/sourmash-4.0.1.dev19%2Bg5c450d7.tar.gz,Sphinx==3.5.3,sphinxcontrib-applehelp==1.0.2,sphinxcontrib-devhelp==1.0.2,sphinxcontrib-htmlhelp==1.0.3,sphinxcontrib-jsmath==1.0.1,sphinxcontrib-napoleon==0.7,sphinxcontrib-qthelp==1.0.3,sphinxcontrib-serializinghtml==1.1.4,testpath==0.4.4,tornado==6.1,traitlets==5.0.5,urllib3==1.26.4,wcwidth==0.2.5,webencodings==0.5.1
tox: docs - invoke sphinx-build to build the HTML docs
  docs run-test-pre: PYTHONHASHSEED='2116944744'
  docs run-test: commands[0] | sphinx-build -d /Users/runner/work/sourmash/sourmash/.tox/docs_doctree doc /Users/runner/work/sourmash/sourmash/.tox/docs_out --color -bhtml
  Running Sphinx v3.5.3

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I was able to reproduce the error with:

mamba create -n sourmash_dev tox-conda rust git compilers pandoc python==3.8
pip install -r requirements.txt
cd doc
make