Add macOS, Windows and Python 3.9 to test matrix

Author: amotl

.github/workflows/main.yml

@@ -8,8 +8,8 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest]
-        python-version: [3.6, 3.7, 3.8]
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        python-version: [3.6, 3.7, 3.8, 3.9]
 
     steps:
       - uses: actions/checkout@master