Update ci.yml

Author: singjc

.github/workflows/ci.yml

@@ -1,6 +1,8 @@
 name: continuous-integration
 
-on: [push]
+on:
+  push:
+  pull_request:
 
 jobs:
   test:
@@ -33,4 +35,4 @@ jobs:
 
     - name: Test
       run: |
-        python -m pytest -n auto tests/
+        python -m pytest -n auto tests/