Add windows to build matrix

Author: terriko

.github/workflows/pythonapp.yml

@@ -5,9 +5,10 @@ on: [push, pull_request]
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
+         os: [ubuntu-latest, windows-latest]
          python: [3.6, 3.7]
     steps:
     - uses: actions/checkout@v1