"TypeError: 'generator' object is not reversible" during RTD builds

Update a PR will the latest changes from master, and the RTD build will fail with the following error: (example build)

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/sphinx/cmd/build.py", line 304, in build_main
    app.build(args.force_all, filenames)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/sphinx/application.py", line 341, in build
    self.builder.build_update()
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 347, in build_update
    len(to_build))
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 360, in build
    updated_docnames = set(self.read())
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 468, in read
    self._read_serial(docnames)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 490, in _read_serial
    self.read_doc(docname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 534, in read_doc
    doctree = read_doc(self.app, self.env, self.env.doc2path(docname))
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/sphinx/io.py", line 318, in read_doc
    pub.publish()
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/docutils/core.py", line 219, in publish
    self.apply_transforms()
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/docutils/core.py", line 200, in apply_transforms
    self.document.transformer.apply_transforms()
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/sphinx/transforms/__init__.py", line 90, in apply_transforms
    Transformer.apply_transforms(self)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/docutils/transforms/__init__.py", line 171, in apply_transforms
    transform.apply(**kwargs)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/sphinx/transforms/__init__.py", line 245, in apply
    apply_source_workaround(n)
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/sphinx/util/nodes.py", line 94, in apply_source_workaround
    for classifier in reversed(node.parent.traverse(nodes.classifier)):
TypeError: 'generator' object is not reversible

Exception occurred:
  File "/home/docs/checkouts/readthedocs.org/user_builds/ivado-medical-imaging/envs/949/lib/python3.7/site-packages/sphinx/util/nodes.py", line 94, in apply_source_workaround
    for classifier in reversed(node.parent.traverse(nodes.classifier)):
TypeError: 'generator' object is not reversible
The full traceback has been saved in /tmp/sphinx-err-1ck_ok_w.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!

[dyt811]

Okay. Gonna try 4.2.0 as per https://github.com/readthedocs/readthedocs.org/blob/master/requirements/pip.txt#L15 said and see how it goes.

[joshuacwnewton]

So I would suggest we take a closer look at why we’re pinning Sphinx to such a low version number.

It looks like this was done in https://github.com/ivadomed/ivadomed/pull/429, with more context in https://github.com/ivadomed/ivadomed/issues/428.