relax an assertion in packing external call args (#2526)

the number of args passed could be somewhere in between
base args + 0 kwargs and
base args + all kwargs

vyper/old_codegen/external_call.py

@@ -121,7 +121,7 @@ def _external_call_helper(
         gas = "gas"
 
     # sanity check
-    assert len(contract_sig.args) == len(args_lll)
+    assert len(contract_sig.base_args) <= len(args_lll) <= len(contract_sig.args)
 
     if context.is_constant() and contract_sig.mutability not in ("view", "pure"):
         # TODO is this already done in type checker?