ci: run Python workflow only when relevant files change

Limit GitHub Actions workflow triggers to changes in .py files,
requirements.txt, setup.py, or pyproject.toml to avoid unnecessary builds.

.github/workflows/ci.yml

@@ -4,9 +4,19 @@ on:
   push:
     branches:
       - master
+    paths:
+      - '**.py'
+      - 'requirements.txt'
+      - 'pyproject.toml'
+      - 'setup.py'
   pull_request:
     branches:
       - master
+    paths:
+      - '**.py'
+      - 'requirements.txt'
+      - 'pyproject.toml'
+      - 'setup.py'
 
 jobs:
   build: