CI: Add testing on macOS and Windows

Author: amotl

.github/workflows/main.yml

@@ -16,6 +16,11 @@ jobs:
       matrix:
         os: ["ubuntu-latest"]
         python-version: ["3.11"]
+        include:
+          - os: "macos-latest"
+            python-version: "3.11"
+          - os: "windows-latest"
+            python-version: "3.11"
 
     env:
       OS: ${{ matrix.os }}