def _get_environ_vars(self):
        # type: () -> Iterable[Tuple[str, str]]
        """Returns a generator with all environmental vars with prefix PIP_"""
        for key, val in os.environ.items():
            should_be_yielded = (
                key.startswith("PIP_") and
                key[4:].lower() not in self._ignore_env_names
            )
            if should_be_yielded:
                yield key[4:].lower(), val