add python 3.9 to the test

Author: xwings

.github/workflows/build-ci.yml

@@ -11,7 +11,7 @@ jobs:
       matrix:
         #os: [windows-2019, macos-10.15, ubuntu-18.04, ubuntu-20.04]
         os: [windows-2019, ubuntu-18.04, ubuntu-20.04]
-        python-version: [3.7, 3.8]
+        python-version: [3.7, 3.8, 3.9]
         exclude:
           - os: ubuntu-20.04
             python-version: 3.7