Loads configuration from configuration files

def _load_config_files(self):
        # type: () -> None
        """Loads configuration from configuration files
        """
        config_files = dict(self._iter_config_files())
        if config_files[kinds.ENV][0:1] == [os.devnull]:
            logger.debug(
                "Skipping loading configuration files due to "
                "environment's PIP_CONFIG_FILE being os.devnull"
            )
            return

        for variant, files in config_files.items():
            for fname in files:
                # If there's specific variant set in `load_only`, load only
                # that variant, not the others.
                if self.load_only is not None and variant != self.load_only:
                    logger.debug(
                        "Skipping file '%s' (variant: %s)", fname, variant
                    )
                    continue

                parser = self._load_file(variant, fname)

                # Keeping track of the parsers used
                self._parsers[variant].append((fname, parser))