class_docstr: The ``trove_classifiers`` package is the official way of validating classifiers,
  however this package might not be always available.
  As a workaround we can still download a list from PyPI.
  We also don't want to be over strict about it, so simply skipping silently is an
  option (classifiers will be validated anyway during the upload to PyPI).
code: def __call__(self, value: str) -> bool:
      if self.downloaded is False or self._skip_download is True:
          return True
  
      if os.getenv("NO_NETWORK") or os.getenv(
          "VALIDATE_PYPROJECT_NO_NETWORK"
      ):
          self.downloaded = False
          msg = (
              "Install ``trove-classifiers`` to ensure proper validation. "
              "Skipping download of classifiers list from PyPI (NO_NETWORK)."
          )
          _logger.debug(msg)
          return True
  
      if self.downloaded is None:
          msg = (
              "Install ``trove-classifiers`` to ensure proper validation. "
              "Meanwhile a list of classifiers will be downloaded from PyPI."
          )
          _logger.debug(msg)
          try:
              self.downloaded = set(
                  _download_classifiers().splitlines()
              )
          except Exception:
              self.downloaded = False
              _logger.debug(
                  "Problem with download, skipping validation"
              )
              return True
  
      return value in self.downloaded or value.lower().startswith(
          "private ::"
      )
  
imports: import logging
  import os
  import re
  import string
  import typing
  from itertools import chain
label_desc: Use image node_id 3 for calling the _TroveClassifier obj's underlying member method code with example usage: obj.__call__(value) and returns: unknown, True, True, True