Cancel concurrent runs (#35)

.github/workflows/pytest.yml

@@ -6,6 +6,12 @@ on:
   pull_request:
     branches: ['*']
 
+# When this workflow is queued, automatically cancel any previous running
+# or pending jobs from the same branch
+concurrency:
+  group: tests-${{ github.ref }}
+  cancel-in-progress: true
+
 defaults:
   run:
     shell: bash -l {0}