Use Python 3.9 final in CI

Author: srittau

.github/workflows/test-and-lint.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9.0-rc.2]
+        python-version: [3.6, 3.7, 3.8, 3.9]
       fail-fast: false
 
     steps: