explicitly use member on 3.13

pyenzyme/units/units.py

@@ -1,6 +1,7 @@
 from copy import deepcopy
-from enum import Enum, member
+from enum import Enum
 from functools import partial
+import sys
 
 from pydantic import model_validator
 
@@ -49,10 +50,18 @@ def set_scale(unit: _BaseUnit, scale: int) -> _BaseUnit:
 class Prefix(Enum):
     """Enumeration for unit prefixes with corresponding scales."""
 
-    k = member(partial(set_scale, scale=3))
-    m = member(partial(set_scale, scale=-3))
-    u = member(partial(set_scale, scale=-6))
-    n = member(partial(set_scale, scale=-9))
+    if sys.version_info >= (3, 13):
+        from enum import member
+
+        k = member(partial(set_scale, scale=3))
+        m = member(partial(set_scale, scale=-3))
+        u = member(partial(set_scale, scale=-6))
+        n = member(partial(set_scale, scale=-9))
+    else:
+        k = partial(set_scale, scale=3)
+        m = partial(set_scale, scale=-3)
+        u = partial(set_scale, scale=-6)
+        n = partial(set_scale, scale=-9)
 
     def __mul__(self, other: _BaseUnit) -> _BaseUnit:
         """Multiply prefix with a BaseUnit.