Add PyPy compatibility
diff --git a/.github/workflows/compile_test.yml b/.github/workflows/compile_test.yml
index 8e206fb..a3fee7c 100644
--- a/.github/workflows/compile_test.yml
+++ b/.github/workflows/compile_test.yml
@@ -4,12 +4,16 @@
 
 jobs:
   build:
-    runs-on: ubuntu-18.04
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        python-version: [ '3.5', '3.6', '3.7', '3.8', '3.9' ]
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        python-version: [
+          '3.5', '3.6', '3.7', '3.8', '3.9',
+          'pypy-3.5', 'pypy-3.6', 'pypy-3.7',
+        ]
         
-    name: Python ${{ matrix.python-version }}
+    name: Python ${{ matrix.python-version }} on ${{ matrix.os }}
 
     steps:
       - uses: actions/checkout@v1