CI: Run test target only on branch main

This hopefully prevents that on pull requests, the tasks will be running
twice.

Author: amotl

.github/workflows/test.yml

@@ -2,10 +2,12 @@ name: Test
 
 on:
   pull_request:
+    branches: [ main ]
     paths:
       - '.github/workflows/test.yml'
       - '**.py'
   push:
+    branches: [ main ]
     paths:
       - '.github/workflows/test.yml'
       - '**.py'