[TODO] Enable PR builds in RTD

[webknjaz]

@jaraco we've discussed this in comments to other PRs/issues and you seemed to be on board with the idea. Except that the only annoying part is that it cannot be done via a config file but only using the UI.
I thought I'd create a dedicated issue for this task so it wouldn't get lost again.

[webknjaz]

@jaraco we've faced an actual problem caused by not having this enabled. It happened 3 weeks ago (#2584) but only got noticed 5 days ago (#2614) and I only saw that issue & put 1+1 today.
I think this checkbox should be enabled ASAP allowing us to prevent this from happening more proactively.

[jaraco]

The checkbox is already checked.

I did not change it. So there must be more work to be done.

[jaraco]

I've unchecked the box, hit save, checked the box, and hit save again. Maybe that will help.

[webknjaz]

Sometimes the GH webhooks break. There's a tab to resync them. Or check in the repo settings whether the corresponding hook actually sends PR events to RTD.

[webknjaz]

Check https://readthedocs.org/dashboard/setuptools/integrations/ (there's a resync button under the specific hook).

And also check https://github.com/pypa/setuptools/settings/hooks -> Ctrl+F for "readthedocs" and see if the Pull Requests checkbox is on.

[jaraco]

In the hooks settings, I changed "Just the 'push' event" to "send me everything" (even though I want them to send RTD everything). Let's see how that works.