docs: Fix docs build

- Fix: Pin docs build requirement `requests` to `<2.30`.

Refs:
- psf/cachecontrol#292
- psf/requests#6437
- https://github.com/psf/requests/blob/main/HISTORY.md#2300-2023-05-03
- https://urllib3.readthedocs.io/en/stable/changelog.html#id3
  or https://github.com/urllib3/urllib3/blob/main/CHANGES.rst#200-2023-04-26
  ("Removed" sub-section)

docs/requirements.txt

@@ -2,3 +2,7 @@ sphinx==6.1.2
 furo==2022.12.7
 sphinx-toolbox==3.4.0
 urwid==2.1.2
+
+# `sphinx-toolbox` requires `cachecontrol`
+# but see https://github.com/ionrock/cachecontrol/issues/292
+requests<2.30