ci: fix test workflow

Author: okunator

.github/workflows/tests.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        python-version: [3.7, 3.10]
+        python-version: ["3.7", "3.10"]
         os: [ubuntu-latest]
     runs-on: ${{ matrix.os }}
     steps: