[3.8] gh-115349: Pin theme to fix code snippets (GH-115351)

[ambv]

Pin theme to fix code snippets
(cherry picked from commit 3fcea41)

• Issue: Code snippets in dark theme are unreadable #115349

[hugovk]

Built locally:

[ambv]

@hugovk I appreciate it but this was about the dark theme, no?

[hugovk]

Yes, we're pinning to the last version before the dark theme was added.

This the minimal fix, because the dark theme requires Sphinx 3 for proper syntax highlighting, and the 3.8/3.9 docs use Sphinx 2.4.4.

We can look into bumping Sphinx to 3+, but it will likely also require bumping other dependencies, and for security branches I felt it would be easier/safer to pin to an older python-docs-theme.

[ambv]

Ah, got it. Right. Okay then, let's go.