Added tox to run on macOS

Author: soimkim

.github/workflows/pull-request.yml

@@ -14,10 +14,11 @@ jobs:
       envPAT: ${{ secrets.GITHUB_TOKEN }}
 
   build:
-    runs-on: ubuntu-latest
     strategy:
       matrix:
         python-version: [3.8, 3.11]
+        os: [ubuntu-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v3
     - name: Set up Python ${{ matrix.python-version }}