ci: run the test suite on the 3 major OSes

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

Author: darosior

.github/workflows/python-package.yml

@@ -7,12 +7,16 @@ on:
     branches: [ master ]
 
 jobs:
-  build:
-    runs-on: ubuntu-latest
+  tests:
     strategy:
       matrix:
         python-version: [3.6, 3.7, 3.8, 3.9]
+        os:
+          - ubuntu-latest
+          - macOS-latest
+          - windows-latest
 
+    runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}