CI: Use Ubuntu 22.04 to keep supporting Python 3.7

Author: amotl

.github/workflows/test.yml

@@ -32,7 +32,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: ["ubuntu-latest"]
+        os: ["ubuntu-22.04"]
         python-version: [
           '3.7', '3.13',
           'pypy-3.9', 'pypy-3.10',