CI: Test against Windows and macOS in addition to Ubuntu

Author: str4d

.github/workflows/ci.yml

@@ -8,9 +8,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        # We don't need to test across multiple platforms yet
-        # os: [ubuntu-latest, windows-latest, macOS-latest]
-        os: [ubuntu-latest]
+        os: [ubuntu-latest, windows-latest, macOS-latest]
 
     steps:
       - uses: actions/checkout@v2