Fix CI

Author: shinaoka

.github/workflows/pytest.yml

@@ -14,9 +14,6 @@ jobs:
     strategy:
       matrix:
         include:
-          - os: ubuntu-20.04
-            numpy-version: 1.16
-            python-version: 3.6
           - os: ubuntu-latest
             numpy-version: auto
             python-version: 3.9