Pin JInja 2

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.