Allow reusing anchors in YAML

YAML Spec permits anchor name reuse, but ruaml.yaml by default errors for them.
https://stackoverflow.com/questions/39013993/parse-a-yaml-with-duplicate-anchors-in-python

src/check_jsonschema/loaders/instance/yaml.py

@@ -1,8 +1,12 @@
 import typing as t
+import warnings
 
 import ruamel.yaml
+from ruamel.yaml.error import ReusedAnchorWarning
 
-_yaml = ruamel.yaml.YAML(typ="safe")
+warnings.simplefilter("ignore", ReusedAnchorWarning)
+
+_yaml = ruamel.yaml.YAML(typ="safe", pure=True)
 
 # ruamel.yaml parses timestamp values into datetime.datetime values which differs from
 # JSON which parses timestamps as strings. Turn off this feature.

src/check_jsonschema/loaders/schema/readers.py

@@ -2,16 +2,20 @@
 
 import json
 import typing as t
+import warnings
 
 import identify
 import ruamel.yaml
+from ruamel.yaml.error import ReusedAnchorWarning
 
 from check_jsonschema import utils
 
 from ...cachedownloader import CacheDownloader
 from ..errors import SchemaParseError
 
-yaml = ruamel.yaml.YAML(typ="safe")
+warnings.simplefilter("ignore", ReusedAnchorWarning)
+
+yaml = ruamel.yaml.YAML(typ="safe", pure=True)
 
 
 def _json_load_schema(schema_location: str, fp: t.IO) -> dict:

tests/unit/test_loaders.py

@@ -55,11 +55,11 @@ def test_schemaloader_local_yaml_data(tmp_path, filename):
   a:
     type: object
     properties:
-      b:
+      b: &anchor
         type: array
         items:
           type: integer
-      c:
+      c: &anchor
         type: string
 """
     )
@@ -130,10 +130,10 @@ def test_instanceloader_yaml_data(tmp_path, filename, default_ft):
     f.write_text(
         """\
 a:
-  b:
+  b: &anchor
    - 1
    - 2
-  c: d
+  c: &anchor d
 """
     )
     loader = InstanceLoader([str(f)], default_filetype=default_ft)