Adjust GitHub Actions workflow image version to accommodate Python 3.7.

Author: lapets

.github/workflows/lint-test-cover-docs.yml

@@ -6,7 +6,7 @@ on:
   workflow_call: # If invoked by build-publish-sign-release workflow.
 jobs:
   lint_test_cover_docs:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: ['3.7', '3.8', '3.9', '3.10', '3.11', '3.12']