Call pip in QProcess worker.

def _call_pip(self, name=None, prefix=None, extra_args=None,
                  callback=None):
        """Call pip in QProcess worker."""
        cmd_list = self._pip_cmd(name=name, prefix=prefix)
        cmd_list.extend(extra_args)

        process_worker = ProcessWorker(cmd_list, pip=True, callback=callback)
        process_worker.sig_finished.connect(self._start)
        self._queue.append(process_worker)
        self._start()

        return process_worker