limit runner to ubuntu-latest python3.8

Author: lazyoracle

.github/workflows/notebook_test.yml

@@ -12,8 +12,8 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-20.04, ubuntu-18.04, macos-latest, windows-latest]
-        python-version: [3.6, 3.7, 3.8]
+        os: [ubuntu-latest]
+        python-version: [3.8]
     env:
       OS: ${{ matrix.os }}
       PYTHON: ${{ matrix.python-version }}