test: GitHub Actions: Test on Python 3.9

From python: [3.6, 3.7, 3.8] --> python: [3.6, 3.8, 3.9] because if things work on Python 3.6 and 3.8 then they should work on 3.7.

https://www.python.org/downloads/release/python-390/
PR-URL: #2230
Reviewed-By: Shelley Vohr <shelley.vohr@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
       max-parallel: 15
       matrix:
         node: [10.x, 12.x, 14.x]
-        python: [3.6, 3.7, 3.8]
+        python: [3.6, 3.8, 3.9]
         os: [macos-latest, ubuntu-latest, windows-latest]
     runs-on: ${{ matrix.os }}
     steps: