Stop using private API jsonschema

This library relied on a private API of jsonschema to map a validation
error to the right `OCPPError`. As of jsonschema==4.19.0, the private
API has changed.[^1]. Using the latest release of jsonschema with this
library caused an `ImportError`.

This commit removes the dependency on the private API.

[^1]: python-jsonschema/jsonschema@9313230

ocpp/messages.py

@@ -9,7 +9,6 @@
 from typing import Callable, Dict, Union
 
 from jsonschema import Draft4Validator
-from jsonschema import _validators as SchemaValidators
 from jsonschema.exceptions import ValidationError as SchemaValidationError
 
 from ocpp.exceptions import (
@@ -226,16 +225,20 @@ def validate_payload(message: Union[Call, CallResult], ocpp_version: str) -> Non
     try:
         validator.validate(message.payload)
     except SchemaValidationError as e:
-        if e.validator == SchemaValidators.type.__name__:
+        if e.validator == "type":
+            # if e.validator == SchemaValidators.type.__name__:
             raise TypeConstraintViolationError(
                 details={"cause": e.message, "ocpp_message": message}
             )
-        elif e.validator == SchemaValidators.additionalProperties.__name__:
+        elif e.validator == "additionalProperties":
+            # elif e.validator == SchemaValidators.additionalProperties.__name__:
             raise FormatViolationError(
                 details={"cause": e.message, "ocpp_message": message}
             )
-        elif e.validator == SchemaValidators.required.__name__:
+        elif e.validator == "required":
+            # elif e.validator == SchemaValidators.required.__name__:
             raise ProtocolError(details={"cause": e.message})
+
         elif e.validator == "maxLength":
             raise TypeConstraintViolationError(
                 details={"cause": e.message, "ocpp_message": message}