Set up CI for macOS and Windows (#17)

Author: kazk

.github/workflows/test.yml

@@ -3,10 +3,12 @@ on: [push, pull_request]
 
 jobs:
   test:
-    runs-on: ubuntu-latest
     strategy:
+      fail-fast: false
       matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
         python-version: [3.6, 3.7, 3.8]
+    runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python ${{ matrix.python-version }}