Fix cached property import

Author: jhavl

roboticstoolbox/robot/ETS.py

@@ -6,7 +6,6 @@
 """
 
 from collections import UserList
-from functools import lru_cache, cached_property
 from numpy import (
     pi,
     where,
@@ -44,9 +43,19 @@
 from spatialmath.base import getvector
 from spatialmath import SE3
 from typing import Union, overload, List, Set
+from sys import version_info
 
 ArrayLike = Union[list, ndarray, tuple, set]
 
+py_ver = version_info
+
+if version_info >= (3, 9):
+    from functools import cached_property
+
+    c_property = cached_property
+else:
+    c_property = property
+
 
 class BaseETS(UserList):
     def __init__(self, *args):
@@ -238,7 +247,7 @@ def jindex_set(self) -> Set[int]:  #
         """
         return set([self[j].jindex for j in self.joint_idx()])  # type: ignore
 
-    @cached_property
+    @c_property
     def jindices(self) -> ndarray:
         """
         Get an array of joint indices
@@ -255,7 +264,7 @@ def jindices(self) -> ndarray:
         """
         return array([self[j].jindex for j in self.joints()])  # type: ignore
 
-    @cached_property
+    @c_property
     def qlim(self):
         r"""
         Joint limits