Ignore non-str $ids for *deprecated* RefResolver resolution.

The new referencing behavior is more correct (which is why it exists),
but also means that even though `RefResolver` was/is untouched, it can
be called now with more subschemas than previously, and in some cases
that tickles this code to blow up (see the closed issue for a specific
example).

We simply now ignore non-strs, as doing so is no more wrong than this
code used to be.

Closes: #1085

jsonschema/tests/test_validators.py

@@ -2328,6 +2328,15 @@ def test_helpful_error_message_on_failed_pop_scope(self):
             resolver.pop_scope()
         self.assertIn("Failed to pop the scope", str(exc.exception))
 
+    def test_pointer_within_schema_with_different_id(self):
+        """
+        See #1085.
+        """
+        schema = validators.Draft7Validator.META_SCHEMA
+        resolver = validators._RefResolver("", schema)
+        validator = validators.Draft7Validator(schema, resolver=resolver)
+        self.assertFalse(validator.is_valid({"maxLength": "foo"}))
+
 
 def sorted_errors(errors):
     def key(error):

jsonschema/validators.py

@@ -1021,9 +1021,10 @@ def _find_in_subschemas(self, url):
             return None
         uri, fragment = urldefrag(url)
         for subschema in subschemas:
-            target_uri = self._urljoin_cache(
-                self.resolution_scope, subschema["$id"],
-            )
+            id = subschema["$id"]
+            if not isinstance(id, str):
+                continue
+            target_uri = self._urljoin_cache(self.resolution_scope, id)
             if target_uri.rstrip("/") == uri.rstrip("/"):
                 if fragment:
                     subschema = self.resolve_fragment(subschema, fragment)