Cancel running jobs when a new commit is pushed to a PR

Author: ddelange

.github/workflows/tests.yml

@@ -9,6 +9,10 @@ on:
     branches:
       - master
 
+concurrency:  # https://stackoverflow.com/questions/66335225#comment133398800_72408109
+  group: ${{ github.workflow }}-${{ github.ref || github.run_id }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   test-platforms:
     # NOTE: this matrix is for testing various combinations of Python and OS