Run tests on Python 3.9 (#107)

Author: itziakos

.github/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
   tests:
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.6, 3.7, 3.8, 3.9]
         os: [windows-2019]
     runs-on: ${{ matrix.os }}
     steps: