Fix an error by python 3.10

Author: jungtaekkim

.github/workflows/pytest.yml

@@ -7,11 +7,11 @@ jobs:
     strategy:
       matrix:
         python-version:
-          - 3.6
-          - 3.7
-          - 3.8
-          - 3.9
-          - 3.10
+          - '3.6'
+          - '3.7'
+          - '3.8'
+          - '3.9'
+          - '3.10'
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2