docs: def valid_env_vars() -> bool:
      """Validate that required env vars exist.
  
      :returns: True if required env vars exist.
  
      .. versionadded:: 0.0.12
      """
      for envvar in _REQUIRED_ENV_VARS:
          try:
              _check_env_var(envvar)
          except KeyError as ex:
              LOG.error(ex)
              sys.exit(1)
      return True