CI: add CPython 3.9 to test matrix.

Author: whitequark

.github/workflows/main.yaml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ['3.6', '3.7', '3.8', pypy3]
+        python-version: ['3.6', '3.7', '3.8', '3.9', pypy3]
     steps:
     - name: Check out source code
       uses: actions/checkout@v2