ci: run py3.12, temporary on all pushes

.github/workflows/matrix-tests.yml

@@ -2,15 +2,13 @@
 # For regular pushes, we just test the baseline Python version, but when pushing
 # to main or release, we want to exercise the full matrix.
 name: Run Matrix Tests
-on:
-  push:
-    branches: [main, release]
+on: push
 jobs:
   test:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest]
-        python-version: ["3.8", "3.9", "3.10", "3.11"]
+        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
         exclude:
           # We're not compatible with Python 3.11 on Windows yet.
           # We get a compilation error about libxml2 when building lxml.