Bug 1755635

Description of problem:
The python-ipykernel package is failing many tests in Rawhide, which I discovered when I looked into building a new version.  It looks like jupyter_client cannot start a manager when stdout refers to /dev/null, at least in mock.  Here's a transcript from a mock shell:

$ python3
Python 3.8.0b4 (default, Aug 30 2019, 00:00:00) 
[GCC 9.2.1 20190827 (Red Hat 9.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> from subprocess import STDOUT
>>> from jupyter_client import manager
/usr/lib/python3.8/site-packages/jupyter_client/manager.py:81: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if self._kernel_spec is None and self.kernel_name is not '':
>>> KM, KC = manager.start_new_kernel(startup_timeout=120, stdout=open(os.devnull), stderr=STDOUT)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.8/site-packages/jupyter_client/manager.py", line 472, in start_new_kernel
    kc.wait_for_ready(timeout=startup_timeout)
  File "/usr/lib/python3.8/site-packages/jupyter_client/blocking/client.py", line 120, in wait_for_ready
    raise RuntimeError('Kernel died before replying to kernel_info')
RuntimeError: Kernel died before replying to kernel_info

On the other hand, leaving stdout and stderr at their default values works:

>>> KM, KC = manager.start_new_kernel(startup_timeout=120)
/usr/lib/python3.8/site-packages/jupyter_client/manager.py:81: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if self._kernel_spec is None and self.kernel_name is not '':
>>> KM
<jupyter_client.manager.KernelManager object at 0x7faa7ff28400>
>>> KC
<jupyter_client.blocking.client.BlockingKernelClient object at 0x7faa7db69eb0>

Setting stderr=STDOUT and leaving stdout alone also works.  Redirecting stdout as shown above succeeds on an F-30 machine with python3-jupyter-client-5.2.3-7.fc32.noarch.

Version-Release number of selected component (if applicable):
python3-jupyter-client-5.2.3-10.fc32.noarch

How reproducible:
Always

Steps to Reproduce:
1. Run the python code above
2.
3.

Actual results:
The manager creation step fails when stdout points to /dev/null.

Expected results:
The manager creation step should succeed.

Additional info: