Revert "testing on windows, linux and mac"

This reverts commit a36816cc7e946117247af0bb16de42c279545c33.

.github/workflows/test.yml

@@ -6,10 +6,9 @@ jobs:
   test:
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
         python: [3.7, 3.8, 3.9]
       fail-fast: false
-    runs-on: ${{ matrix.os }}
+    runs-on: ubuntu-latest
     timeout-minutes: 10
     steps:
       - uses: actions/checkout@v2