CI: Only run on the oldest and newest supported versions of Python

In order to save resources.

Author: amotl

.github/workflows/test.yml

@@ -31,8 +31,8 @@ jobs:
       matrix:
         os: ["ubuntu-20.04"]
         python-version: [
-          '3.6', '3.7', '3.8', '3.9', '3.10', '3.11',
-          'pypy-3.6', 'pypy-3.7', 'pypy-3.8', 'pypy-3.9', 'pypy-3.10',
+          '3.6', '3.11',
+          'pypy-3.6', 'pypy-3.10',
         ]
     steps:
       - uses: actions/checkout@v3