Upgrade to Python 3.9

Author: cclauss

Diff for: .github/workflows/build.yml

@@ -12,7 +12,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
         with:
-          python-version: "3.8"
+          python-version: "3.9"
       - uses: actions/cache@v2
         with:
           path: ~/.cache/pip