Fix an error for pytest.yml

.github/workflows/pytest.yml

@@ -7,13 +7,12 @@ jobs:
     strategy:
       matrix:
         python-version:
-          - '3.6'
           - '3.7'
           - '3.8'
           - '3.9'
           - '3.10'
           - '3.11'
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2