Read the docs is failing to build

[drvinceknight]

This isn't the simplest thing to fix because I can't seem to force a build of a given branch no readthedocs.io but I'm working on it.

Error message:

Running Sphinx v1.8.5

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/envs/master/lib/python3.7/site-packages/sphinx/config.py", line 368, in eval_config_file
    execfile_(filename, namespace)
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/envs/master/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 150, in execfile_
    exec_(code, _globals)
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/checkouts/master/docs/conf.py", line 55, in <module>
    import axelrod
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/checkouts/master/axelrod/__init__.py", line 13, in <module>
    from axelrod.classifier import Classifiers
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/checkouts/master/axelrod/classifier.py", line 15, in <module>
    import yaml
ModuleNotFoundError: No module named 'yaml'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/envs/master/lib/python3.7/site-packages/sphinx/cmd/build.py", line 303, in build_main
    args.tags, args.verbosity, args.jobs, args.keep_going)
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/envs/master/lib/python3.7/site-packages/sphinx/application.py", line 201, in __init__
    self.config = Config.read(self.confdir, confoverrides or {}, self.tags)
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/envs/master/lib/python3.7/site-packages/sphinx/config.py", line 203, in read
    namespace = eval_config_file(filename, tags)
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/envs/master/lib/python3.7/site-packages/sphinx/config.py", line 380, in eval_config_file
    raise ConfigError(msg % traceback.format_exc())
sphinx.errors.ConfigError: There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/envs/master/lib/python3.7/site-packages/sphinx/config.py", line 368, in eval_config_file
    execfile_(filename, namespace)
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/envs/master/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 150, in execfile_
    exec_(code, _globals)
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/checkouts/master/docs/conf.py", line 55, in <module>
    import axelrod
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/checkouts/master/axelrod/__init__.py", line 13, in <module>
    from axelrod.classifier import Classifiers
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/checkouts/master/axelrod/classifier.py", line 15, in <module>
    import yaml
ModuleNotFoundError: No module named 'yaml'


Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/envs/master/lib/python3.7/site-packages/sphinx/config.py", line 368, in eval_config_file
    execfile_(filename, namespace)
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/envs/master/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 150, in execfile_
    exec_(code, _globals)
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/checkouts/master/docs/conf.py", line 55, in <module>
    import axelrod
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/checkouts/master/axelrod/__init__.py", line 13, in <module>
    from axelrod.classifier import Classifiers
  File "/home/docs/checkouts/readthedocs.org/user_builds/axelrod/checkouts/master/axelrod/classifier.py", line 15, in <module>
    import yaml
ModuleNotFoundError: No module named 'yaml'