validators: avoid urllib.request at import-time

This change improves jsonschema import time by ~50ms.

On my end calling:

python -X importtime -c "import jsonschema"

* Before: 234ms
* After: 173ms

Author: gudnimg

jsonschema/validators.py

@@ -9,7 +9,6 @@
 from operator import methodcaller
 from typing import TYPE_CHECKING
 from urllib.parse import unquote, urldefrag, urljoin, urlsplit
-from urllib.request import urlopen
 from warnings import warn
 import contextlib
 import json
@@ -1225,6 +1224,7 @@ def resolve_remote(self, uri):
             result = requests.get(uri).json()
         else:
             # Otherwise, pass off to urllib and assume utf-8
+            from urllib.request import urlopen
             with urlopen(uri) as url:  # noqa: S310
                 result = json.loads(url.read().decode("utf-8"))