remove macos and windows from test matrix

Author: DanielRuf

.github/workflows/main.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [windows-latest, ubuntu-latest, macos-latest]
+        os: [ubuntu-latest]
         node: [10, 12, 13]
     steps:
       - uses: actions/checkout@v1