Use system font stack for all code

[hugovk]

Follow on from https://github.com/python/python-docs-theme/pull/176.

I missed one of the code font styles.

It's subtle but can be seen in this table:

• https://docs.python.org/3.13/library/asyncio-task.html#waiting-primitives

The left column has the old font (e.g. asyncio.FIRST_COMPLETED), but the right has the new one (e.g. the ALL_COMPLETED link).

Preview:

• https://python-docs-theme-previews--186.org.readthedocs.build/en/186/library/asyncio-task.html#waiting-primitives