CI: Use ubuntu-22.04 to support Python 3.7

Author: amotl

.github/workflows/tests.yml

@@ -23,7 +23,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [
-          "ubuntu-latest",
+          "ubuntu-22.04",
           "macos-12",
           "macos-latest",
         ]