Support Unix systems without O_NOFOLLOW (#463)

Acquiring a lock uses the `O_NOFOLLOW` flag when calling `open`.
However, this flag is not available on all platforms, such as GraalPy,
causing an `AttributeError` to be raised when attempting to acquire a
lock. According to the Python docs:

> The above constants are extensions and not present if they are not
> defined by the C library.

This handles such platforms by checking for the presence of the
`O_NOFOLLOW` flag first.

Author: mwilliamson

src/filelock/_unix.py

@@ -38,7 +38,10 @@ class UnixFileLock(BaseFileLock):
 
         def _acquire(self) -> None:
             ensure_directory_exists(self.lock_file)
-            open_flags = os.O_RDWR | os.O_TRUNC | os.O_NOFOLLOW
+            open_flags = os.O_RDWR | os.O_TRUNC
+            o_nofollow = getattr(os, "O_NOFOLLOW", None)
+            if o_nofollow is not None:
+                open_flags |= o_nofollow
             if not Path(self.lock_file).exists():
                 open_flags |= os.O_CREAT
             fd = os.open(self.lock_file, open_flags, self._context.mode)