pythonGH-85633: Fix pathlib test failures on filesystems without worl…

…d-write.

Replace `umask(0)` with `umask(2)`, and `umask(0o022)` with `umask(0o026)`,
so the created files are not world-writable. In the latter case, we remove
read permission to check that permissions for 'others' can still be set.

Lib/test/test_pathlib/test_pathlib.py

@@ -1605,18 +1605,18 @@ def test_absolute_posix(self):
     )
     @needs_posix
     def test_open_mode(self):
-        old_mask = os.umask(0)
+        old_mask = os.umask(2)
         self.addCleanup(os.umask, old_mask)
         p = self.cls(self.base)
         with (p / 'new_file').open('wb'):
             pass
         st = os.stat(self.parser.join(self.base, 'new_file'))
-        self.assertEqual(stat.S_IMODE(st.st_mode), 0o666)
-        os.umask(0o022)
+        self.assertEqual(stat.S_IMODE(st.st_mode), 0o664)
+        os.umask(0o026)
         with (p / 'other_new_file').open('wb'):
             pass
         st = os.stat(self.parser.join(self.base, 'other_new_file'))
-        self.assertEqual(stat.S_IMODE(st.st_mode), 0o644)
+        self.assertEqual(stat.S_IMODE(st.st_mode), 0o640)
 
     @needs_posix
     def test_resolve_root(self):