Unit test jobs are failing with ImportError: cannot import name 'engine' from 'flake8'

[Nate Johnston]

Neutron unit test CI jobs are failing with the following error:

=========================
Failures during discovery
=========================
--- import errors ---
Failed to import test module: neutron.tests.unit.hacking.test_checks
Traceback (most recent call last):
  File "/usr/lib/python3.7/unittest/loader.py", line 436, in _find_test_path
    module = self._get_module_from_name(name)
  File "/usr/lib/python3.7/unittest/loader.py", line 377, in _get_module_from_name
    __import__(name)
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/unit/hacking/test_checks.py", line 15, in <module>
    from flake8 import engine
ImportError: cannot import name 'engine' from 'flake8' (/home/zuul/src/opendev.org/openstack/neutron/.tox/py37/lib/python3.7/site-packages/flake8/__init__.py)

Example: https://e859f0a6f5995c9142c5-a232ce3bdc50fca913ceba9a1c600c62.ssl.cf5.rackcdn.com/572767/23/check/openstack-tox-py37/1d036e0/job-output.txt

Looks like flake8 no longer has an engine but they had kept the api for backward compatibility [1], perhaps they broke it somehow.

[1] based on comment in https://gitlab.com/pycqa/flake8/blob/master/src/flake8/api/legacy.py#L3

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/699243

[Nate Johnston (nate-johnston)]

See related change https://review.opendev.org/#/c/699021/ which pins the version of hacking, which fixes this issue. But this will remain a problem if we ever unpin, so let's schedule a good time to figure out the dependency issues with unpinning.

[OpenStack Infra (hudson-openstack)]

Change abandoned by Brian Haley (<email address hidden>) on branch: master
Review: https://review.opendev.org/699243
Reason: Not required, file changed after move to hacking>3