CI: run unit tests in debug mode with python 3.11

Python 3.12 does not work with current aiohttp, see
aio-libs/aiohttp#7229

It is currently possible to build a wheel using aiohttp==3.9.0b0.
However, unit tests fail in that case, because TestLNTransport::test_loop stalls.

.cirrus.yml

@@ -20,7 +20,7 @@ task:
            ELECTRUM_PYTHON_VERSION: 3.11
        - name: "unittests: Tox Python 3 debug mode"
          env:
-           ELECTRUM_PYTHON_VERSION: 3
+           ELECTRUM_PYTHON_VERSION: 3.11
            # enable additional checks:
            PYTHONASYNCIODEBUG: "1"
            PYTHONDEVMODE: "1"