fix: quote "3.10" in ci

otherwise it looks for 3.1

Reference:

- actions/setup-python#249 (comment)

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [2.7, 3.7, 3.8, 3.9, 3.10]
+        python-version: ["2.7", "3.7", "3.8", "3.9", "3.10"]
 
     steps:
     - uses: actions/checkout@v2