build executables on GitHub Actions with Python 3.8

Python 3.9 is incompatible with Windows 7, so using a lower
Python version maybe allows those files to run on Windows 7.

.github/workflows/executables.yml

@@ -17,7 +17,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v2
       with:
-        python-version: 3.9
+        python-version: 3.8
 
     - name: Build executable
       run: |