Revert "CI: run on pull requests"

This reverts commit 666a464.

It turned out to be a bit abusive, since Github doesn't deduplicate
such actions; every time I pushed to a PR-eligible branch, it would
build the whole project TWICE. That's just a waste of resources.

We need to get some kind of test suite which is run before the sdist
action is deemed a success, and run *that* on each commit; then we
could consider not running the full bdist suite on every commit, but
only on PRs?

.github/workflows/python_build.yaml

@@ -1,6 +1,6 @@
 name: Build
 
-on: [pull_request, push]
+on: [push]
 
 jobs:
   sdist: