pin sphinx-youtube to 1.3.0 for sphinx5 compat

Author: mmcky

environment.yml

@@ -13,7 +13,7 @@ dependencies:
     - sphinxext-rediraffe==0.2.7
     - git+https://github.com/executablebooks/sphinx-exercise@main
     - ghp-import==2.1.0
-    - sphinxcontrib-youtube==1.4.1
+    - sphinxcontrib-youtube==1.3.0  #Version 1.3.0 is required as quantecon-book-theme is only compatible with sphinx<=5
     - sphinx-togglebutton==0.3.2
     # Github Actions Bug
     - requests