ci: run main tests on macOS and Windows, too

.github/workflows/check.yml

@@ -8,11 +8,15 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [macos-latest, ubuntu-latest, windows-latest]
     steps:
       - uses: actions/checkout@v4
       # No caching as we're expected to run rarely
       - name: Install dependencies and run checks
+        shell: bash
         run: |
           set -euxo pipefail
           python3 -m pip install pre-commit