Attempt to narrow CI/test trigger to just code changes

Looking at
https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request
I *think* this might be the approach to take. Consider this an experimental
change that will very likely get rolled back.

See #2404

.github/workflows/pythonpackage.yml

@@ -1,6 +1,11 @@
 name: Test Textual module
 
-on: [pull_request]
+on:
+  pull_request:
+    paths:
+      - '**.py'
+      - '**.pyi'
+      - '**.css'
 
 jobs:
   build: