Revert "fix: optional type var for Reference with python 3.13+"

This reverts commit e14fdf0.

Author: azmeuk

scim2_models/reference.py

@@ -1,4 +1,3 @@
-import sys
 from collections import UserString
 from typing import Any
 from typing import Generic
@@ -12,11 +11,7 @@
 
 from .utils import UNION_TYPES
 
-# TypeVar with default parameter is available in Python 3.13+
-if sys.version_info >= (3, 13):
-    ReferenceTypes = TypeVar("ReferenceTypes", default=Any)
-else:  # pragma: no cover
-    ReferenceTypes = TypeVar("ReferenceTypes")
+ReferenceTypes = TypeVar("ReferenceTypes")
 
 URIReference = NewType("URIReference", str)
 ExternalReference = NewType("ExternalReference", str)