Add missing urllib.request import to models.link (#13726)

This code worked only by accident as urllib.request is imported
elsewhere. This was noticed by the Ty type checker.

Author: danielhollas

src/pip/_internal/models/link.py

@@ -7,6 +7,7 @@
 import posixpath
 import re
 import urllib.parse
+import urllib.request
 from collections.abc import Mapping
 from dataclasses import dataclass
 from typing import (