[BugFix] fix permission denied error when lock file is placed in `/tm…

…p` (#317)

src/filelock/_unix.py

@@ -4,6 +4,7 @@
 import sys
 from contextlib import suppress
 from errno import ENOSYS
+from pathlib import Path
 from typing import cast
 
 from ._api import BaseFileLock
@@ -35,7 +36,9 @@ class UnixFileLock(BaseFileLock):
 
         def _acquire(self) -> None:
             ensure_directory_exists(self.lock_file)
-            open_flags = os.O_RDWR | os.O_CREAT | os.O_TRUNC
+            open_flags = os.O_RDWR | os.O_TRUNC
+            if not Path(self.lock_file).exists():
+                open_flags |= os.O_CREAT
             fd = os.open(self.lock_file, open_flags, self._context.mode)
             with suppress(PermissionError):  # This locked is not owned by this UID
                 os.fchmod(fd, self._context.mode)