Only trigger CI builds on master branch and pull requests

If we trigger on every branch we will have redundant CI runs for
branches with pull requests.

.github/workflows/main.yml

@@ -1,5 +1,9 @@
 name: CI
-on: [push, pull_request]
+on:
+  push:
+    branches: ["master"]
+  pull_request:
+
 jobs:
   tests:
     name: "Python ${{ matrix.python-version }}"