build: Fix binary for Windows 7

Python 3.9 doesn't support Windows 7. Build with Python 3.8 instead.

.github/workflows/build.yml

@@ -26,10 +26,10 @@ jobs:
         uses: actions/checkout@v2
         with:
           submodules: true
-      - name: Set up Python 3.9
+      - name: Set up Python 3.8
         uses: actions/setup-python@v2
         with:
-          python-version: 3.9
+          python-version: 3.8
       - if: matrix.os == 'ubuntu-16.04'
         run: sudo apt-get install -y libyaml-dev
       - name: Install PyInstaller