Fix #33243: Move return out of finally block in niconico downloader

Python 3.14 emits SyntaxWarning for 'return' in a 'finally' block.
Move 'return success' after the try/finally to eliminate the warning
while preserving identical behavior (success is always set before
the finally block runs).

Added test_niconico_downloader.py verifying no SyntaxWarning on import.

test/test_niconico_downloader.py

@@ -0,0 +1,24 @@
+#!/usr/bin/env python
+# coding: utf-8
+from __future__ import unicode_literals
+
+import subprocess
+import sys
+import unittest
+
+
+class TestNiconicoDownloader(unittest.TestCase):
+    def test_no_syntax_warning_return_in_finally(self):
+        """Issue #33243: 'return' in a 'finally' block triggers SyntaxWarning in Python 3.14+"""
+        result = subprocess.run(
+            [sys.executable, '-W', 'error::SyntaxWarning', '-c',
+             'from youtube_dl.downloader.niconico import NiconicoDmcFD'],
+            capture_output=True, text=True)
+        self.assertEqual(
+            result.returncode, 0,
+            'Importing niconico downloader raised SyntaxWarning '
+            '(return in finally block):\n' + result.stderr)
+
+
+if __name__ == '__main__':
+    unittest.main()

youtube_dl/downloader/niconico.py

@@ -63,4 +63,4 @@ class NiconicoDmcFD(FileDownloader):
                 with heartbeat_lock:
                     timer[0].cancel()
                     download_complete = True
-            return success
+        return success