Set a library-specific user agent when automatically retrieving $refs.

This behavior is (already) deprecated, but oddly there are "real world"
webservers which seem to have allowed the `requests` default user agent
(i.e. respond correctly) but not the `urllib` default vague `Python/3.x`
one (i.e. respond with a 403 or whatever).

Since it seems polite to set this anyhow, we may as well fix behavior
for anyone who happens to be encountering such a webserver.

Author: Julian

Diff for: CHANGELOG.rst

@@ -1,3 +1,8 @@
+v4.18.6
+=======
+
+* Set a ``jsonschema`` specific user agent when automatically retrieving remote references (which is deprecated).
+
 v4.18.5
 =======
 

Diff for: jsonschema/tests/test_deprecations.py

@@ -1,7 +1,11 @@
-from unittest import TestCase
-import importlib
+from contextlib import contextmanager
+from io import BytesIO
+from unittest import TestCase, mock
+import importlib.metadata
+import json
 import subprocess
 import sys
+import urllib.request
 
 import referencing.exceptions
 
@@ -16,8 +20,9 @@ def test_version(self):
 
         message = "Accessing jsonschema.__version__ is deprecated"
         with self.assertWarnsRegex(DeprecationWarning, message) as w:
-            from jsonschema import __version__  # noqa: F401
+            from jsonschema import __version__
 
+        self.assertEqual(__version__, importlib.metadata.version("jsonschema"))
         self.assertEqual(w.filename, __file__)
 
     def test_validators_ErrorTree(self):
@@ -357,3 +362,44 @@ def test_cli(self):
             capture_output=True,
         )
         self.assertIn(b"The jsonschema CLI is deprecated ", process.stderr)
+
+    def test_automatic_remote_retrieval(self):
+        """
+        Automatic retrieval of remote references is deprecated as of v4.18.0.
+        """
+        ref = "http://bar#/$defs/baz"
+        schema = {
+            "$schema": "https://json-schema.org/draft/2020-12/schema",
+            "$defs": {"baz": {"type": "integer"}},
+        }
+
+        if "requests" in sys.modules:  # pragma: no cover
+            self.addCleanup(
+                sys.modules.__setitem__, "requests", sys.modules["requests"],
+            )
+        sys.modules["requests"] = None
+
+        @contextmanager
+        def fake_urlopen(request):
+            self.assertIsInstance(request, urllib.request.Request)
+            self.assertEqual(request.full_url, "http://bar")
+
+            # Ha ha urllib.request.Request "normalizes" header names and
+            # Request.get_header does not also normalize them...
+            (header, value), = request.header_items()
+            self.assertEqual(header.lower(), "user-agent")
+            self.assertEqual(
+                value, "python-jsonschema (deprecated $ref resolution)",
+            )
+            yield BytesIO(json.dumps(schema).encode("utf8"))
+
+        validator = validators.Draft202012Validator({"$ref": ref})
+
+        message = "Automatically retrieving remote references "
+        patch = mock.patch.object(urllib.request, "urlopen", new=fake_urlopen)
+
+        with patch, self.assertWarnsRegex(DeprecationWarning, message):
+            self.assertEqual(
+                (validator.is_valid({}), validator.is_valid(37)),
+                (False, True),
+            )

Diff for: jsonschema/validators.py

@@ -103,8 +103,10 @@ def _validates(cls):
 
 
 def _warn_for_remote_retrieve(uri: str):
-    from urllib.request import urlopen
-    with urlopen(uri) as response:
+    from urllib.request import Request, urlopen
+    headers = {"User-Agent": "python-jsonschema (deprecated $ref resolution)"}
+    request = Request(uri, headers=headers)
+    with urlopen(request) as response:
         warnings.warn(
             "Automatically retrieving remote references can be a security "
             "vulnerability and is discouraged by the JSON Schema "