Build on multiple OS

Author: John Tompkins

.github/workflows/pr-ci.yaml

@@ -16,7 +16,8 @@ jobs:
       matrix:
         python: [3.6, 3.7, 3.8]
         java: [8, 11]
-    runs-on: ubuntu-latest
+        os: [ ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python }}