Docs build: Add upper bounds to transitive dependencies

Doc/constraints.txt

@@ -0,0 +1,23 @@
+# Direct dependencies of Sphinx
+babel<3
+colorama<0.5
+imagesize<1.5
+Jinja2<3.2
+packaging<24
+# Pygments==2.15.0 breaks CI
+Pygments<2.16,!=2.15.0
+requests<3
+snowballstemmer<3
+sphinxcontrib-applehelp<1.1
+sphinxcontrib-devhelp<1.1
+sphinxcontrib-htmlhelp<2.1
+sphinxcontrib-jsmath<1.1
+sphinxcontrib-qthelp<1.1
+sphinxcontrib-serializinghtml<1.2
+
+# Direct dependencies of Jinja2 (Jinja is a dependency of Sphinx, see above)
+MarkupSafe<2.2
+
+# Direct dependencies of sphinx-lint
+polib<1.3
+regex<2024

Doc/requirements.txt

@@ -13,3 +13,7 @@ sphinxext-opengraph==0.7.5
 # The theme used by the documentation is stored separately, so we need
 # to install that as well.
 python-docs-theme>=2022.1
+
+# We have upper bounds on our transitive dependencies in constraints.txt
+# To avoid new releases unexpectedly breaking our build
+-c constraints.txt