fix: method flush() in iostream.py

[ghost]

When shutting down, the flush() method does not work correctly.

if self.pub_thread.thread.is_alive():
AttributeError: 'NoneType' object has no attribute 'thread'

[C 02:15:42.905 NotebookApp] Shutdown confirmed
[I 02:15:42.908 NotebookApp] Shutting down 2 kernels
Error in atexit._run_exitfuncs:
Traceback (most recent call last):
File "/Users/valentin/brew/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/logging/init.py", line 2033, in shutdown
h.flush()
File "/Users/valentin/.pyenv3/tensorflow/lib/python3.7/site-packages/absl/logging/init.py", line 891, in flush
self._current_handler.flush()
File "/Users/valentin/.pyenv3/tensorflow/lib/python3.7/site-packages/absl/logging/init.py", line 785, in flush
self.stream.flush()
File "/Users/valentin/.pyenv3/tensorflow/lib/python3.7/site-packages/ipykernel/iostream.py", line 341, in flush
if self.pub_thread.thread.is_alive():
AttributeError: 'NoneType' object has no attribute 'thread'
[I 02:15:43.531 NotebookApp] Kernel shutdown: d514bdbe-8879-44de-b4d9-01d160a5be94
[I 02:15:43.534 NotebookApp] Kernel shutdown: 7579b41f-76f6-4a7c-8dd8-c35bf90dbbba

[minrk]

Thanks!