📦 NEW: add os to matrix

Author: wasdee

.github/workflows/build.yml

@@ -4,9 +4,11 @@ on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
+      fail-fast: false
       matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
         python-version: [ '3.6', '3.7', '3.8', '3.9', '3.10-dev' ]
     steps:
       - uses: actions/checkout@v2.3.4