CI: drop 3.13 for self-tests (ndonnx does not build)

Author: ev-br

.github/workflows/tests.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ['3.11', '3.12', '3.13']
+        python-version: ['3.11', '3.12']
         numpy-version: ['1.26', 'latest', 'dev']
         exclude:
           - python-version: '3.13'