ci(tests) Add Python 3.15

Adds 3.15 to the tests matrix alongside 3.10-3.14; 3.14 is kept
because the matrix covers more than 3.14 and the oldest version. The
suite passed on 3.15.0 locally (`uv run py.test -q`, 102 passed).

See also: https://devguide.python.org/versions/

Author: tony

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ['3.10', '3.11', '3.12', '3.13', '3.14']
+        python-version: ['3.10', '3.11', '3.12', '3.13', '3.14', '3.15']
         docutils-version: ['0.20', '0.22.4']
         pytest-version: ['8', '9']
         exclude: