fix syntax for python3.10

Author: Jonathan Kliem

.github/workflows/main.yml

@@ -14,7 +14,7 @@ jobs:
             fail-fast: false
             matrix:
                 os: [ubuntu-latest]
-                python-version: [3.6, 3.7, 3.8, 3.9, 3.10]
+                python-version: ['3.6', '3.7', '3.8', '3.9', '3.10']
                 pari-version: ['pari-2.9.4', 'pari-2.9.5', 'pari-2.11.0', 'pari-2.11.0', 'pari-2.11.2', 'pari-2.11.3', 'pari-2.11.4', 'pari-2.13.0', 'snapshot']
         env:
           LC_ALL: C