fix: added runtime dependnecy referencing>=0.28.4" (#846)

Signed-off-by: Jan Kowalleck <jan.kowalleck@gmail.com>

Author: jkowalleck

pyproject.toml

@@ -73,12 +73,13 @@ py-serializable =  "^2.0.0"
 sortedcontainers = "^2.4.0"
 license-expression = "^30"
 jsonschema = { version = "^4.18", extras=['format'], optional=true }
+referencing = { version = ">=0.28.4", optional=true }  # in sync with the transitive dependency of `jsonschema`
 lxml = { version=">=4,<7", optional=true }
 typing_extensions = { version="^4.6", python = "<3.13"}  # for `@deprecated` - which was added in v4.5 but this version appesrs to be broken...
 
 [tool.poetry.extras]
-validation = ["jsonschema", "lxml"]
-json-validation = ["jsonschema"]
+validation = ["jsonschema", "referencing", "lxml"]
+json-validation = ["jsonschema", "referencing"]
 xml-validation = ["lxml"]
 
 [tool.poetry.group.dev.dependencies]