Fix workflow trigger for Python package version check

`pull_request_target` runs against the base and doesn't work with paths,
so it's wrong. It is more targeted toward protecting workflows from
rogue PRs from forks.

This isn't an issue for this workflow because it doesn't use any
secrets, apart from ephemeral ones generated through OIDC flows, and
in any event it's protected from fork runs by the `if` check in
`check-python-package-versions.yaml` on line 11.

Author: vcschapp

.github/workflows/check-python-package-versions.yaml

@@ -1,7 +1,7 @@
 name: Check Python package version numbers
 
 on:
-  pull_request_target:
+  pull_request:
     paths:
       - '**/pyproject.toml'
       - 'packages/**/__about__.py'