fix: self.is_valid_type requires

Co-authored-by: alpharush <0xalpharush@protonmail.com>

slither/detectors/variables/possible_const_state_variables.py

@@ -46,7 +46,7 @@ class ConstCandidateStateVars(AbstractDetector):
 
     @staticmethod
     def _valid_candidate(v):
-        return _is_valid_type(v) and not (v.is_constant or v.is_immutable)
+        return self._is_valid_type(v) and not (v.is_constant or v.is_immutable)
 
     # https://solidity.readthedocs.io/en/v0.5.2/contracts.html#constant-state-variables
     valid_solidity_function = [