API documentation not rendered on readthedocs

[cdunklau]

The docs should have the autogenerated API docs, but they don't render on RTD. I just tried running the tox env, and it rendered locally, so it's not a global issue.

Might have a similar cause as Yelp/bravado#285

Edit: I think I found it. The RTD build isn't installing deps, so autodoc fails, but doesn't fail the build because it's not running with -E. Migrating to .readthedocs.yml is probably the best way to fix this.