Build wheels for Python 3.9.

.github/workflows/pythonpublish.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       matrix:
         os: [macos-latest, windows-latest]
-        python-version: ['3.5', '3.6', '3.7', '3.8']
+        python-version: ['3.5', '3.6', '3.7', '3.8', '3.9']
         include:
           - os: ubuntu-latest
             python-version: '3.x'