ci: run tests on linux, mac and windows

Author: woile

.github/workflows/pythonpackage.yml

@@ -4,12 +4,12 @@ on: [pull_request]
 
 jobs:
   python-check:
-    runs-on: ubuntu-latest
     strategy:
       max-parallel: 4
       matrix:
         python-version: [3.6, 3.7, 3.8]
-
+        platform: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.platform }}
     steps:
     - uses: actions/checkout@v2
       with: