FIX: add sphinx-youtube to execution testing

Author: mmcky

.github/workflows/execution.yml

@@ -23,7 +23,7 @@ jobs:
         shell: bash -l {0}
         run: |
           conda install anaconda
-          pip install jupyter-book sphinx-multitoc-numbering quantecon-book-theme sphinxext-rediraffe sphinx_tojupyter
+          pip install jupyter-book sphinx-multitoc-numbering quantecon-book-theme sphinxext-rediraffe sphinx_tojupyter sphinxcontrib-youtube
       - name: Build Lectures (+ Execution Checks)
         shell: bash -l {0}
         run: jb build lectures --path-output=./ -W --keep-going