Previews in pull requests

[hugovk]

Fixes https://github.com/python/python-docs-theme/issues/70.

Same as https://github.com/python/python-docs-theme/pull/105, hopefully will now trigger a Read the Docs build.

[hugovk]

Nope, will re-open the original!

It worked on my fork, will compare settings:

• https://github.com/hugovk/python-docs-theme/pull/1
• https://readthedocs.org/dashboard/hugovk-python-docs-theme/webhooks/