gh-143010: Prevent a TOCTOU issue by only calling open once (#143011)

* gh-143010: Prevent a TOCTOU issue by gh-143010: Prevent a TOCTOU issue by only calling open once

RDM: per  AZero13's research the 'x' option did not exist when this code was written,  This
modernization can thus drop the fd trick in _create_carefully and just use open with 'x' to achieve the same goal more securely.

Co-authored-by: sobolevn <mail@sobolevn.me>