`mailbox._create_carefully` has a possible toctou error

[AZero13]

Bug report

Bug description:

We should just reuse the fd and not double close.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-143010: Prevent a TOCTOU issue by only calling open once #143011
• [3.13] gh-143010: Prevent a TOCTOU issue by only calling open once (GH-143011) #143079
• [3.14] gh-143010: Prevent a TOCTOU issue by only calling open once (GH-143011) #143080