The readthedocs build is broken

[exarkun]

Recently readthedocs builds of the docs have begun to fail like this:

Running Sphinx v1.8.5
loading translations [en]... done
making output directory...
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 55 source files that are out of date
updating environment: 55 added, 0 changed, 0 removed
reading sources... [  1%] CODE_OF_CONDUCT
reading sources... [  3%] Installation/install-on-desert-island
reading sources... [  5%] Installation/install-on-linux
reading sources... [  7%] Installation/install-on-windows
reading sources... [  9%] Installation/install-tahoe
reading sources... [ 10%] about-tahoe
reading sources... [ 12%] accepting-donations
reading sources... [ 14%] anonymity-configuration
reading sources... [ 16%] architecture
reading sources... [ 18%] backdoors
reading sources... [ 20%] backupdb
reading sources... [ 21%] build/build-pyOpenSSL
reading sources... [ 23%] cautions
reading sources... [ 25%] configuration
reading sources... [ 27%] contributing
reading sources... [ 29%] convergence-secret
reading sources... [ 30%] debian
reading sources... [ 32%] developer-guide
reading sources... [ 34%] donations
reading sources... [ 36%] expenses
reading sources... [ 38%] filesystem-notes
reading sources... [ 40%] frontends/CLI
reading sources... [ 41%] frontends/FTP-and-SFTP
reading sources... [ 43%] frontends/download-status
reading sources... [ 45%] frontends/webapi
/home/docs/checkouts/readthedocs.org/user_builds/tahoe-lafs/envs/1149/lib/python3.7/site-packages/recommonmark/parser.py:65: UserWarning: Container node skipped: type=document
  warn("Container node skipped: type={0}".format(mdnode.t))

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/tahoe-lafs/envs/1149/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/tahoe-lafs/envs/1149/lib/python3.7/site-packages/sphinx/application.py", line 341, in build
    self.builder.build_update()
  File "/home/docs/checkouts/readthedocs.org/user_builds/tahoe-lafs/envs/1149/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/tahoe-lafs/envs/1149/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/tahoe-lafs/envs/1149/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/tahoe-lafs/envs/1149/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/tahoe-lafs/envs/1149/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/tahoe-lafs/envs/1149/lib/python3.7/site-packages/sphinx/io.py", line 318, in read_doc
    pub.publish()
  File "/home/docs/checkouts/readthedocs.org/user_builds/tahoe-lafs/envs/1149/lib/python3.7/site-packages/docutils/core.py", line 219, in publish
    self.apply_transforms()
  File "/home/docs/checkouts/readthedocs.org/user_builds/tahoe-lafs/envs/1149/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/tahoe-lafs/envs/1149/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/tahoe-lafs/envs/1149/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/tahoe-lafs/envs/1149/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/tahoe-lafs/envs/1149/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/tahoe-lafs/envs/1149/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-ojuum04j.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!

Since this CI job is required for merging it *seems* like this may be due to changes in our dependencies.

This currently makes CI red and blocks most contributors from merging anything.

[GitHub <noreply@…>]

In b4bed1d/trunk:

Merge pull request #1151 from tahoe-lafs/3831-sphinx

Tell RTD how to install Sphinx

Fixes: ticket:3831