expand test matrix to windows, macos and pypy-37

.github/workflows/test.yml

@@ -4,10 +4,14 @@ on: [ push, pull_request ]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}-latest
     strategy:
       matrix:
-        python-version: [ 3.6, 3.7, 3.8, 3.9 ]
+        os: [ubuntu, macos, windows]
+        python-version: [ 3.6, 3.7, 3.8, 3.9, pypy-3.7 ]
+        exclude:
+          - os: windows
+            python-version: pypy-3.7
 
     steps:
       - uses: actions/checkout@v2
@@ -21,4 +25,4 @@ jobs:
         run: pip install .
 
       - name: Unit tests
-        run: python tests/nest_test.py
+        run: python tests/nest_test.py