Bump pydata sphinx theme version

[dstansby]

This bumps the pydata-sphinx-theme version to the same one currently being used on the Matplotlib docs (0.15.x).

[timhoffm]

Why are we pinning to 0.15.x? Are there any known incompatibilities either 0.16.x?

Also, more generally, what is our update strategy? When do we plan to update and which checks should be performed?

[dstansby]

My thinking was to be consistent with matplotlib/matplotlib, which is pinned to 0.15.x at the moment.

In terms of a update strategy, I think we should revive matplotlib/mpl-sphinx-theme#84 and put a pydata-sphinx-theme pin in the theme itself, which would avoid having to do this pinning across all the website repositories. Then we can set up dependabot in mpl-sphinx-theme to handle bumping the pydata version.

[dstansby]

I've opened matplotlib/mpl-sphinx-theme#106 to move the pinning of pydata-sphinx-theme to mpl-sphinx-theme.

[dstansby]

After matplotlib/mpl-sphinx-theme#106, I think this is redundant.