updated os.open to preserve mode (#211)

Author: jahrules

docs/changelog.rst

@@ -1,5 +1,9 @@
 Changelog
 =========
+v3.10.4 (2023-03-24)
+--------------------
+- Update os.open to preserve mode= for certain edge cases. by :user:`jahrules`.
+
 v3.10.3 (2023-03-23)
 --------------------
 - Fix permission issue - by :user:`jahrules`.

src/filelock/_unix.py

@@ -32,7 +32,7 @@ class UnixFileLock(BaseFileLock):
 
         def _acquire(self) -> None:
             open_flags = os.O_RDWR | os.O_CREAT | os.O_TRUNC
-            fd = os.open(self._lock_file, open_flags)
+            fd = os.open(self._lock_file, open_flags, self._mode)
             try:
                 os.chmod(fd, self._mode)
             except PermissionError: