Add test with Python 3.9 in CI, reformat workflow

Author: flashcode

.github/workflows/ci.yml

@@ -1,15 +1,23 @@
 name: CI
 
-on: [push, pull_request]
+on:
+  - push
+  - pull_request
 
 jobs:
 
   build:
 
     strategy:
       matrix:
-        os: [ubuntu-20.04, macos-latest]
-        python-version: [3.6, 3.7, 3.8]
+        os:
+          - ubuntu-20.04
+          - macos-latest
+        python-version:
+          - "3.6"
+          - "3.7"
+          - "3.8"
+          - "3.9"
 
     name: Python ${{ matrix.python-version }} on ${{ matrix.os }}
     runs-on: ${{ matrix.os }}