ci: also test on macOS and Windows with ONLY Python 3.9

Don't want our build matrix to get TOO ridiculous, but one build on each
other platform is good enough for sanity checks.

Author: dgw

.github/workflows/ci.yml

@@ -15,17 +15,24 @@ on:
 jobs:
   tests:
     name: Test suite
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
+        os: [ubuntu-latest]
         python-version:
           - 3.6
           - 3.7
           - 3.8
           - 3.9
+        include:
+          # experimental testing on non-Linux platforms
+          - python-version: 3.9
+            os: windows-latest
+          - python-version: 3.9
+            os: macos-latest
     steps:
       - uses: actions/checkout@v2
-      - name: Set up Python ${{ matrix.python-version }}
+      - name: Set up Python ${{ matrix.python-version }} on ${{ runner.os }}
         uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python-version }}