test(linkcheck): fix ruff B010 in timeout retry test

Use a closure counter instead of dynamic attrs on TCPServer (Ruff B010,
ty unresolved-attribute).

Made-with: Cursor

Author: root

tests/test_builders/test_build_linkcheck.py

@@ -1269,6 +1269,7 @@ def do_GET(self) -> None:
 )
 def test_retries_after_transient_timeout(app: SphinxTestApp) -> None:
     """linkcheck_retries must retry when the server is slow (issue #14339)."""
+    get_n = [0]
 
     class SlowThenFastHandler(BaseHTTPRequestHandler):
         protocol_version = 'HTTP/1.1'
@@ -1280,9 +1281,8 @@ def do_HEAD(self) -> None:
             self.end_headers()
 
         def do_GET(self) -> None:
-            server = self.server
-            count = getattr(server, '_linkcheck_get_n', 0) + 1
-            setattr(server, '_linkcheck_get_n', count)
+            get_n[0] += 1
+            count = get_n[0]
             if count <= 2:
                 time.sleep(0.15)
             content = b'ok\n'