CI: Run CoW on 3.11 only
It appears `unit-tests.yml` is spawning too many jobs so it appears CoW jobs are being canceled.

@jorisvandenbossche @phofl are you OK with only running `pandas_copy_on_write: "1"` tests on 3.11 (or another singular Python version)?
Huh looks like the CoW job still got canceled. Maybe the number of jobs a workflow can spawn changed recently (or something else is going wrong) > @jorisvandenbossche @phofl are you OK with only running pandas_copy_on_write: "1" tests on 3.11 (or another singular Python version)?

No CoW needs to run on all Python versions. They changed the ref count in CPython between 3.10 and 3.11 and CoW relies on this for warnings.  OK sounds good. Closing