Fix RTD build

.readthedocs.yml

@@ -5,7 +5,9 @@
 version: 2
 
 build:
-  image: latest
+  os: ubuntu-22.04
+  tools:
+    python: "3.9"
 
 sphinx:
   configuration: conf.py

requirements.txt

@@ -6,6 +6,7 @@
 sphinx==3.5.2
 sphinx_rtd_theme==0.5.2
 docutils<0.18
+Jinja2<3.1
 
 # Code tabs extension for GDScript/C#
 # Stay on 1.3.0 until https://github.com/readthedocs/readthedocs-sphinx-search/issues/82 is fixed.
@@ -19,3 +20,10 @@ sphinxext-opengraph==0.4.2
 
 # Full-page search UI for RTD: https://readthedocs-sphinx-search.readthedocs.io
 readthedocs-sphinx-search==0.1.0
+
+# These get pulled in by Sphinx, we need to pin these as higher versions require Sphinx 5.0+.
+sphinxcontrib-applehelp<=1.0.4
+sphinxcontrib-htmlhelp<=2.0.1
+sphinxcontrib-qthelp<=1.0.3
+sphinxcontrib-serializinghtml<=1.1.5
+sphinxcontrib-devhelp<=1.0.2