cross-platform testing

why not?

.github/workflows/python-package.yml

@@ -11,10 +11,11 @@ on:
 
 jobs:
   test:
-    runs-on: Ubuntu-20.04
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
         python-version: [ 3.7, 3.8, 3.9 ]
+        os: [ macos-latest, windows-latest, Ubuntu-20.04]
 
     steps:
       - uses: actions/checkout@v2
@@ -29,7 +30,11 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
 
-      - name: Set up Linux
+      - name: Set up Windows environment
+        if: runner.os == 'Windows'
+        uses: ilammy/msvc-dev-cmd@v1
+
+      - name: Set up Linux environment
         if: runner.os == 'Linux'
         run: |
           echo "CC=gcc-10" >> $GITHUB_ENV