code: def clean_env_before_and_after_tests():
      """Fixture to clean up env variables before and after tests."""
      os.environ.pop("NBFORMAT_VALIDATOR", None)
      yield
      os.environ.pop("NBFORMAT_VALIDATOR", None)
  
imports: from __future__ import annotations
  import json
  import os
  import re
  from copy import deepcopy
  import pytest
  from jsonschema import ValidationError
  import nbformat
  from nbformat import read
  from nbformat.json_compat import VALIDATORS
  from nbformat.validator import isvalid, iter_validate, validate
  from nbformat.warnings import DuplicateCellId, MissingIDFieldWarning
  from .base import TestsBase
label_desc: Use image node_id 1 for calling a global function with example usage: clean_env_before_and_after_tests() without return types