Start using Python 3.9

Author: abdullahselek

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.5, 3.6, 3.7, 3.8, 3.9-dev]
+        python-version: [3.5, 3.6, 3.7, 3.8, 3.9]
         operating-system: [ubuntu-latest, windows-latest, macos-latest]
 
     steps: