ci: run the test matrix on maintenance branches

A push to main runs the matrix, a pull request runs the matrix, and a
maintenance line such as 2.4.x runs nothing. Those branches never reach main
through a pull request, so 2.4.1 was very nearly tagged and published on the
strength of one local run against one Python and one Django.

The branch already carries this, since the workflow that runs on a push is the
one on the pushed branch. This brings it to main so the next maintenance line
inherits it, and the pattern is generic rather than naming 2.4.x.

.github/workflows/test.yml

@@ -5,6 +5,11 @@ on:
   push:
     branches:
       - main
+      # Maintenance lines, 2.4.x and whatever follows. A push to one of these is
+      # a release candidate and deserves the same matrix as main, and it is the
+      # only chance it gets: these branches never reach main through a pull
+      # request, so nothing else would ever run the tests on them.
+      - "*.x"
 
 concurrency:
   group: check-${{ github.ref }}