chore(ci): don't run push & PR on PR branches

This change prevents duplicate runs from occurring when opening a pull request
on GitHub. Before, both the 'push' and 'pull_request' hooks would trigger,
meaning that any push to a branch with an open PR would run the CI pipeline
twice. By filtering the 'push' hook to 'main' and 'develop', we avoid this.

.github/workflows/testing.yml

@@ -1,5 +1,13 @@
 name: Testing Django
-on: [ pull_request, push ] # activates the workflow when there is a push or pull request in the repo
+
+# Activates the workflow when there is a push or pull request in the repo
+on:
+  push:
+    branches:
+      - main
+      - develop
+  pull_request:
+
 jobs:
   test_project:
     runs-on: ubuntu-latest # operating system your code will run on