Why is the documentation build breaking?

[textbook]

Per https://readthedocs.org/projects/py-wlc/builds/ the Sphinx doc builds are failing due to a type issue with source_suffix:

Running Sphinx v1.3b2
making output directory...
loading translations [en]... done
WARNING: the config value 'source_suffix' has type `list', defaults to `str.'

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-wlc/envs/latest/lib/python3.4/site-packages/sphinx/cmdline.py", line 245, in main
    opts.warningiserror, opts.tags, opts.verbosity, opts.jobs)
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-wlc/envs/latest/lib/python3.4/site-packages/sphinx/application.py", line 179, in __init__
    self._init_env(freshenv)
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-wlc/envs/latest/lib/python3.4/site-packages/sphinx/application.py", line 207, in _init_env
    self.env.find_files(self.config)
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-wlc/envs/latest/lib/python3.4/site-packages/sphinx/environment.py", line 434, in find_files
    self.srcdir, config.source_suffix, exclude_matchers=matchers))
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-wlc/envs/latest/lib/python3.4/site-packages/sphinx/util/__init__.py", line 96, in get_matching_docs
    suffixpattern = '*' + suffix
TypeError: Can't convert 'list' object to str implicitly

Exception occurred:
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-wlc/envs/latest/lib/python3.4/site-packages/sphinx/util/__init__.py", line 96, in get_matching_docs
    suffixpattern = '*' + suffix
TypeError: Can't convert 'list' object to str implicitly
The full traceback has been saved in /tmp/sphinx-err-rsqrgeqo.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://bitbucket.org/birkenfeld/sphinx/issues/>. Thanks!