Update two more ValueError messages to use the consistent format established in PR #130835 (commit 968f6e5), which added %T type prefix and normalized should → must for invalid return type errors. These two locations were not covered by the original PR or the subsequent follow-ups #144827 and #144737.

Changes:

• Objects/typeobject.c — slot_sq_length: "len() should return >= 0" → "%T.len() must return >= 0"
• Python/sysmodule.c — sizeof error: "sizeof() should return >= 0" → "%T.sizeof() must return >= 0"

• Issue: Inconsistent error messages when returning the wrong type for the type-conversion magic methods #130821