Upgrade .circleci/config.yml

[cclauss]

Perform various upgrades as prompted by the CircleCI extension for VSCode.

• https://marketplace.visualstudio.com/items?itemName=circleci.circleci

Should Node.js be upgraded as the current version is v22.9.0? --> #11667 (review)

-  codecov: codecov/codecov@3.2.2
-  node: circleci/node@5.1.0
+  codecov: codecov/codecov@5.0.0
+  node: circleci/node@6.3.0

-      - run: pip install --user tox
+      - run: pipx install tox

[cclauss]

Not ready for Python 3.12?

• https://circleci.com/gh/readthedocs/readthedocs.org/28368 and
• https://circleci.com/gh/readthedocs/readthedocs.org/28366

    @property
    def backend(self):
        """Current backend instance."""
        try:
>           return self._local.backend
E           AttributeError: '_thread._local' object has no attribute 'backend'

.tox/ext-theme/lib/python3.12/site-packages/celery/app/base.py:1250: AttributeError