Pin to last ubuntu version that still supports Python 3.7

Author: Bryan Bartley

.github/workflows/python-package.yml

@@ -18,7 +18,7 @@ jobs:
       fail-fast: false
       matrix:
         # Default builds are on Ubuntu
-        os: [ubuntu-latest]
+        os: [ubuntu-22.04]
 #        python-version: [3.7, 3.8, 3.9, pypy-3.7]
         python-version: ['3.7', '3.8', '3.9', '3.10', 'pypy-3.9']
         include: