Docs: pin sphinx to <9.0 (#14067) (#14068)

Pinning to `<9.0` to fix doc building.

Related to python-trio/sphinxcontrib-trio#399.

(cherry picked from commit cdb8366)

Co-authored-by: Bruno Oliveira <bruno@pytest.org>

Author: patchback[bot]

doc/en/requirements.txt

@@ -2,7 +2,8 @@
 pluggy>=1.5.0
 pygments-pytest>=2.5.0
 sphinx-removed-in>=0.2.0
-sphinx>=7
+# Pinning to <9.0 due to https://github.com/python-trio/sphinxcontrib-trio/issues/399.
+sphinx>=7,<9.0
 sphinxcontrib-trio
 sphinxcontrib-svg2pdfconverter
 furo