Add informative CI tests against Python 3.14-dev

Unfortunately, it'll fail for a while until a new version of
"flit-core" is published (hopefully soon: pypa/flit#695 ?).

.github/workflows/tests.yml

@@ -9,7 +9,6 @@ on:
 jobs:
   tests:
     if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.repository
-    continue-on-error: ${{ endsWith(matrix.python-version, '-dev') }}
     strategy:
       fail-fast: false
       matrix:
@@ -23,6 +22,7 @@ jobs:
         - '3.11'
         - '3.12'
         - '3.13'
+        - 3.14-dev
         - pypy-3.10
         include:
         - os: ubuntu-20.04
@@ -34,6 +34,7 @@ jobs:
         - os: macos-13
           python-version: '3.12'
     runs-on: ${{ matrix.os }}
+    continue-on-error: ${{ endsWith(matrix.python-version, '-dev') }}
     steps:
     - name: Checkout repository
       uses: actions/checkout@v4