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