FIX: prettytable for os x testing

Author: mmcky

.github/workflows/execution.yml

@@ -69,7 +69,7 @@ jobs:
         shell: bash -l {0}
         run: |
           conda install anaconda
-          pip install jupyter-book sphinx-multitoc-numbering quantecon-book-theme sphinxext-rediraffe sphinx-tojupyter sphinxcontrib-youtube sphinx-exercise
+          pip install jupyter-book sphinx-multitoc-numbering quantecon-book-theme sphinxext-rediraffe sphinx-tojupyter sphinxcontrib-youtube sphinx-exercise prettytable
       - name: Build Lectures (+ Execution Checks)
         shell: bash -l {0}
         run: jb build lectures --path-output=./ -W --keep-going