ci: test with Python 3.15, allowed to fail

Author: azmeuk

.github/workflows/tests.yaml

@@ -21,10 +21,12 @@ jobs:
   tests:
     name: py${{ matrix.python }} unit tests
     runs-on: ubuntu-24.04
+    continue-on-error: ${{ matrix.python == '3.15' }}
     strategy:
       fail-fast: false
       matrix:
         python:
+          - '3.15'
           - '3.14'
           - '3.13'
           - '3.12'