Use release version of Python 3.9 in tests

Author: tobiasge

.github/workflows/push.yml

@@ -20,7 +20,7 @@ jobs:
         docker_from:
         - '' # use the default of the build script
         - python:3.8-alpine
-        - python:3.9-rc-alpine
+        - python:3.9-alpine
       fail-fast: false
     runs-on: ubuntu-latest
     name: Builds new Netbox Docker Images