Exposing RWF_DONTCACHE and RWF_ATOMIC flags in the os module

[vstinner (Victor Stinner)]

@claydugo

With that in mind, would you accept a PR that exposes RWF_DONTCACHE (Linux 6.14+) and RWF_ATOMIC (Linux 6.13+)?

os.RWF_DONTCACHE was already added to Python 3.15 by expose `RWF_DONTCACHE` constant · Issue #139090 · python/cpython · GitHub.

PR gh-127922 adding RWF_ATOMIC was rejected because:

it’s not even documented by Linux yet

RWF_ATOMIC is now documented in the pwritev2 manual page. So I suggest to propose again a PR adding RWF_ATOMIC. I would support such PR.