Add `os.RWF_ATOMIC` constant

[claydugo]

Feature or enhancement

Proposal:

I opened a thread on Discourse about this and received some support.

See man page for details: https://man7.org/linux/man-pages/man2/writev.2.html

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

#127922

Linked PRs

• gh-143474: Add os.RWF_ATOMIC constant #143475