ci: use released python 3.9

Author: davidhewitt

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.6, 3.7, 3.8, 3.9-dev, pypy3]
+        python-version: [3.6, 3.7, 3.8, 3.9, pypy3]
     steps:
       - uses: actions/checkout@master
       - name: Setup python