CI reactivate py39

Author: MuellerSeb

.github/workflows/main.yml

@@ -25,7 +25,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        python-version: [3.5, 3.6, 3.7, 3.8]
+        python-version: [3.5, 3.6, 3.7, 3.8, 3.9]
 
     steps:
       - uses: actions/checkout@v2
@@ -56,7 +56,7 @@ jobs:
           python -m coveralls --service=github
 
       - uses: actions/upload-artifact@v2
-        if: matrix.os == 'ubuntu-latest' && matrix.python-version == '3.8'
+        if: matrix.os == 'ubuntu-latest' && matrix.python-version == '3.9'
         with:
           path: dist