CI: don't run unit tests on "python:rc", looks like it's unmaintained

The "python:rc" tag on dockerhub has not been updated for 2+ years
and is still at 3.10.0rc2. Even 3.11 has been released for many months now.
see https://hub.docker.com/_/python/tags

.cirrus.yml

@@ -18,8 +18,6 @@ task:
            ELECTRUM_PYTHON_VERSION: 3.10
        - env:
            ELECTRUM_PYTHON_VERSION: 3.11
-       - env:
-           ELECTRUM_PYTHON_VERSION: rc
        - name: Tox Python 3 debug mode
          env:
            ELECTRUM_PYTHON_VERSION: 3