Add OS specific build to PR gate (#141)

Signed-off-by: gaugup <gaugup@microsoft.com>

Author: gaugup

.github/workflows/pythonpackage.yml

@@ -16,10 +16,11 @@ jobs:
     strategy:
       matrix:
         python-version: [3.6, 3.7, 3.8]
+        os: [ubuntu-latest, macos-latest, windows-latest]
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python ${{ matrix.python-version }}
+    - name: Set up Python ${{ matrix.python-version }} ${{ matrix.os }}
       uses: actions/setup-python@v1
       with:
         python-version: ${{ matrix.python-version }}