Pin OS and Python versions in test-pr.yml

CI was broken because `ubuntu-latest` changed from Ubuntu 22.04 to
24.04, and the latter doesn't have Python 3.7. I pinned to using Ubuntu
22.04 so we can continue using Python 3.7.

In addition, I pinned to the specific Python 3.7 patch release to limit
possible breakage.

Author: aphedges

.github/workflows/test-pr.yml

@@ -23,10 +23,10 @@ concurrency:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04 # Needed for Python 3.7
     name: Run tests
     env:
-      PYTHON_VERSION: "3.7"
+      PYTHON_VERSION: 3.7.17
     defaults:
       run:
         shell: bash -leo pipefail {0}