[3.13] GH-85633: Fix pathlib test failures on filesystems without world-write. (GH-122883) (#122979)

GH-85633: Fix pathlib test failures on filesystems without world-write. (GH-122883)

Replace `umask(0)` with `umask(0o002)` so the created files are not
world-writable, and replace `umask(0o022)` with `umask(0o026)` to check
that permissions for 'others' can still be set.
(cherry picked from commit 5f6851152254b4b9d70af4ae5aea3f20965cee28)

Co-authored-by: Barney Gale <barney.gale@gmail.com>