docs: def get_stderr(self, tail=None):
          """
          Returns current total output written to standard error.
          :param tail: Return this number of most-recent lines.
          :return: copy of stderr stream
          """
          if self.finished(): 
              self.join_threads()
          while not self.stderr_q.empty():
              self.stderr_l.append(self.stderr_q.get_nowait())
          if tail is None:
              tail = len(self.stderr_l)
          return _py2_and_3_joiner('\n', self.stderr_l[:tail])