Disable CI for macOS and Windows (#19)

Author: kazk

.github/workflows/test.yml

@@ -6,7 +6,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-latest]
         python-version: [3.6, 3.7, 3.8]
     runs-on: ${{ matrix.os }}
     steps: