python-referencing: Add new package

From the README:

An implementation-agnostic implementation of JSON reference resolution.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

lang/python/python-referencing/test.sh

@@ -0,0 +1,35 @@
+#!/bin/sh
+
+[ "$1" = python3-referencing ] || exit 0
+
+python3 - << 'EOF'
+
+from referencing import Registry, Resource
+import referencing.jsonschema
+
+schema = Resource.from_contents(  # Parse some contents into a 2020-12 JSON Schema
+    {
+        "$schema": "https://json-schema.org/draft/2020-12/schema",
+        "$id": "urn:example:a-202012-schema",
+        "$defs": {
+            "nonNegativeInteger": {
+                "$anchor": "nonNegativeInteger",
+                "type": "integer",
+                "minimum": 0,
+            },
+        },
+    }
+)
+registry = schema @ Registry()  # Add the resource to a new registry
+
+# From here forward, this would usually be done within a library wrapping this one,
+# like a JSON Schema implementation
+resolver = registry.resolver()
+resolved = resolver.lookup("urn:example:a-202012-schema#nonNegativeInteger")
+assert resolved.contents == {
+    "$anchor": "nonNegativeInteger",
+    "type": "integer",
+    "minimum": 0,
+}
+
+EOF