unit test ThreadDataLoader error

[wyli]

Describe the bug

/opt/monai# python -m tests.test_thread_buffer

output:

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/opt/conda/lib/python3.6/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/opt/conda/lib/python3.6/threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "/opt/monai/monai/data/thread_buffer.py", line 45, in enqueue_values
    for src_val in self.src:
TypeError: 'super' object is not iterable
...
----------------------------------------------------------------------
Ran 3 tests in 3.625s

e.g. https://github.com/Project-MONAI/MONAI/runs/2266946281?check_suite_focus=true#step:6:3770