Pin exact version of pydata-sphinx-theme

[dstansby]

Since the goal of this sphinx theme is to maintain a consistent look across multiple documentation builds, it makes sense to pin the exact version of pydata-sphinx-theme to prevent different doc bulids being done with different versions and looking different.

This also allows us to bump this pin only when we've checked there aren't any issues with the next pydata-sphinx-theme version (e.g. see #80 for recent issues).

[timhoffm]

I’m not convinced pinning is really helpful here. We don’t want to spent explicit time on checking that new versions of pydata-Sphinx-theme is still working for us. I think the devdocs can serve as an experimental build for the current version of the documentation environment. it’s more likely that will find problems if the docs are ready available. Only when we see problems, we should temporarily pin the theme. Pragmatically, if we don’t encounter any problems in the devdocs there either are none or they are small enough so that they are bearable in release builds.

[dstansby]

Makes sense - given that we have spotted problems (#80) I think it still makes sense to put this pin in (I want to rebuild some other MPL docs, https://matplotlib.org/cycler/ and https://matplotlib.org/cheatsheets/ to have a consistent theme with matplotlib.org) and then remove it when #80 is fixed?

[dstansby]

Given https://github.com/matplotlib/matplotlib/pull/27658/files has now been merged, I don't think we need to do this pinning any more so I'll close this.