Revert "Fix workflow trigger for Python package version check"

This reverts commit df4cc38.

Reverting this because it was based on a misunderstanding of what
`pull_request_target` does. It seems that `pull_request_target`
(apart from the secret stuff) mostly determines which branch gets to
decide on the workflows to run. We want the target to decide this (as
an extra security precaution), rather than the PR, so it should have
been fine. On further testing anyway, it seems like some other issue
is to blame here.

Author: vcschapp

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

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