Avoid duplicate CI runs: trigger push CI on master only

With push CI on all branches, every push to a branch with an open PR
fired both a push run and a pull_request run - identical 8-job matrices
twice. Branch work is now covered by pull_request only; pushes to
master (merges) still run the full matrix.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: vthorsteinsson

.github/workflows/python-package.yml

@@ -1,10 +1,13 @@
 name: tests
 
 on:
+  # Run push CI on master only; branch work is covered by pull_request.
+  # (Running push CI on all branches would duplicate every run - both
+  # events fire for each push to a branch with an open PR.)
   push:
-    # Note: "**" is required to match branch names containing '/'
-    branches: [ "**" ]
+    branches: [ master ]
   pull_request:
+    # Note: "**" is required to match branch names containing '/'
     branches: [ "**" ]
 
 jobs: