Test against multiple operating systems in CI pipeline

Now that we have operating system resource accessing functionality, we
really want to test against all the major operating systems.

Author: ruohola

.github/workflows/ci.yml

@@ -93,8 +93,11 @@ jobs:
       fail-fast: true
       matrix:
         python-version: ['3.7', '3.8', '3.9', '3.10']
-
-    runs-on: ubuntu-latest
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    defaults:
+      run:
+        shell: bash
+    runs-on: ${{ matrix.os }}
     steps:
       - name: "Checkout code"
         uses: actions/checkout@v2
@@ -136,7 +139,7 @@ jobs:
           pytest --verbose --cov=. --cov-report=xml .
 
       - name: "Upload coverage"
-        if: matrix.python-version == '3.7'
+        if: matrix.python-version == '3.7' && matrix.os == 'ubuntu-latest'
         uses: codecov/codecov-action@f32b3a3741e1053eb607407145bc9619351dc93b  # v2.1.0
         with:
           files: coverage.xml