FIX: fix nightly 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 sphinx-exercise
       - name: Install latex dependencies
         run: |
           sudo apt-get -qq update
@@ -65,7 +65,7 @@ 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 sphinx-exercise
       - name: Install latex dependencies
         run: |
           brew install texlive