Looks like the doc build isn't happy:
/home/circleci/project/lib/matplotlib/gridspec.py:docstring of matplotlib.gridspec:1: WARNING: more than one target found for cross-reference 'Axes': mpl_toolkits.axes_grid1.mpl_axes.Axes, mpl_toolkits.axisartist.axislines.Axes, matplotlib.axes.Axes

@dstansby Thanks, fixed.

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 v3.1.x
$ git pull

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

$ git cherry-pick -m1 0536a669742e88e56a9e080580dbba727c9f33cd

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

$ git commit -am 'Backport PR #14449: Improve docs on gridspec'

1. Push to a named branch :

git push YOURFORK v3.1.x:auto-backport-of-pr-14449-on-v3.1.x

1. Create a PR against branch v3.1.x, I would have named this PR:

"Backport PR #14449 on branch v3.1.x"

And apply the correct labels and milestones.

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

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