gh-149083: Use sentinel for urllib.parse._UNSPECIFIED (GH-149612)

This was added in 3.15; let's use a real sentinel instead of an ad-hoc list object.
(cherry picked from commit 884ac3e)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Author: JelleZijlstra

Lib/urllib/parse.py

@@ -277,7 +277,7 @@ def _hostinfo(self):
         return hostname, port
 
 
-_UNSPECIFIED = ['not specified']
+_UNSPECIFIED = sentinel("_UNSPECIFIED", repr="<not specified>")
 _MISSING_AS_NONE_DEFAULT = False
 
 class _ResultBase: