CI: Run Python build tests on PRs

This is the only regular testing we get on exotic platforms (Windows,
MacOS), and it helps spot changes in the manifest Cargo.toml that only
manifest during Python builds.

.github/workflows/python-wheels.yml

@@ -5,6 +5,7 @@ on:
       branches: [main]
       tags: 'v*'
     workflow_dispatch:
+    pull_request:
 
 jobs:
   build-python-wheels: