Fix doc build

[jasongrout]

Rely on the default of readthedocs installing things with pip, rather than insisting on setuptools (which doesn't work anymore for us).

[jasongrout]

@meeseeksdev please backport to master

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout master
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -m1 c255e99f7433972b8586c187f8f97799fc18b90f

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #3416: Fix doc build'

4. Push to a named branch:

git push YOURFORK master:auto-backport-of-pr-3416-on-master

5. Create a PR against branch master, I would have named this PR:

"Backport PR #3416 on branch master (Fix doc build)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.

[jasongrout]

@meeseeksdev please backport to master

Actually, looks like this is already fixed in master.