Add missing import

I suspect this got lost in a merge somewhere, probably the
combination of 7e533ca7e1 and the
Python 3.12 work in a60fdba1d4.

test/test_inference/test_signature.py

@@ -1,5 +1,5 @@
 from textwrap import dedent
-from operator import ge
+from operator import ge, lt
 import re
 import os