Some minor issues found via static analysis

[tzickel]

Unreachable code https://github.com/celery/billiard/blob/d0be0308a9dfd17c28277bb582edcf009ecd3153/billiard/pool.py#L665 https://github.com/celery/billiard/blob/d0be0308a9dfd17c28277bb582edcf009ecd3153/billiard/pool.py#L1229

Found using cython and https://gist.github.com/tzickel/ea4541b12c1298a8092c3ec5043ef77e

pip install -I --root temp_celery celery
python checkcython.py temp_celery

there are some other minor issues in the other subprojects, since I don't want to spam, you can run this lines and see them all.

[auvipy]

pr welcome and i will review