ci: Fix API doc failure in CI (#4075)

Sphinx 8.2 (see
[changelog](https://www.sphinx-doc.org/en/master/changes/index.html#release-8-2-0-released-feb-18-2025))
seems to have broken our CI. Looks like an incompatibility between it
and the autodoc-typehints extension, so hopefully the two catch up with
one another -- I'll pin sphinx to <8.2 for now.

Author: sentrivana

requirements-docs.txt

@@ -1,5 +1,5 @@
 gevent
 shibuya
-sphinx
+sphinx<8.2
 sphinx-autodoc-typehints[type_comments]>=1.8.0
 typing-extensions