Run more GitHub actions jobs in parallel

Author: Cito

.github/workflows/test-package.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 5
     strategy:
-      max-parallel: 4
+      max-parallel: 6
       matrix:
         python-version: ['3.7', '3.8', '3.9', '3.10', '3.11', 'pypy-3.9']