Autopkgtest fails because Python 3.8's hmc.new() needs digest parameter

https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-focal/focal/amd64/p/python-taskflow/20191104_102436_6fd99@/log.gz

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taskflow.tests.unit.test_engines.ParallelEngineWithProcessTest.test_graph_flow_two_tasks
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Captured traceback:
~~~~~~~~~~~~~~~~~~~
    b'Traceback (most recent call last):'
    b' File "/tmp/autopkgtest.YG0BhS/build.G9O/src/taskflow/tests/unit/test_engines.py", line 867, in test_graph_flow_two_tasks'
    b' engine = self._make_engine(flow)'
    b' File "/tmp/autopkgtest.YG0BhS/build.G9O/src/taskflow/tests/unit/test_engines.py", line 1522, in _make_engine'
    b' return taskflow.engines.load(flow, flow_detail=flow_detail,'
    b' File "/tmp/autopkgtest.YG0BhS/build.G9O/src/taskflow/engines/helpers.py", line 133, in load'
    b' mgr = stevedore.driver.DriverManager('
    b' File "/usr/lib/python3/dist-packages/stevedore/driver.py", line 53, in __init__'
    b' super(DriverManager, self).__init__('
    b' File "/usr/lib/python3/dist-packages/stevedore/named.py", line 78, in __init__'
    b' extensions = self._load_plugins(invoke_on_load,'
    b' File "/usr/lib/python3/dist-packages/stevedore/extension.py", line 203, in _load_plugins'
    b' self._on_load_failure_callback(self, ep, err)'
    b' File "/usr/lib/python3/dist-packages/stevedore/extension.py", line 191, in _load_plugins'
    b' ext = self._load_one_plugin(ep,'
    b' File "/usr/lib/python3/dist-packages/stevedore/named.py", line 156, in _load_one_plugin'
    b' return super(NamedExtensionManager, self)._load_one_plugin('
    b' File "/usr/lib/python3/dist-packages/stevedore/extension.py", line 227, in _load_one_plugin'
    b' obj = plugin(*invoke_args, **invoke_kwds)'
    b' File "/tmp/autopkgtest.YG0BhS/build.G9O/src/taskflow/engines/action_engine/engine.py", line 589, in __init__'
    b' self._task_executor = self._fetch_task_executor(self._options)'
    b' File "/tmp/autopkgtest.YG0BhS/build.G9O/src/taskflow/engines/action_engine/engine.py", line 637, in _fetch_task_executor'
    b' return executor_cls(**kwargs)'
    b' File "/tmp/autopkgtest.YG0BhS/build.G9O/src/taskflow/engines/action_engine/process_executor.py", line 603, in __init__'
    b' self._dispatcher = Dispatcher({}, self._auth_key,'
    b' File "/tmp/autopkgtest.YG0BhS/build.G9O/src/taskflow/engines/action_engine/process_executor.py", line 533, in __init__'
    b' self.challenge_pieces = _encode_message(auth_key, CHALLENGE,'
    b' File "/tmp/autopkgtest.YG0BhS/build.G9O/src/taskflow/engines/action_engine/process_executor.py", line 276, in _encode_message'
    b' message_mac = _calculate_hmac(auth_key, message)'
    b' File "/tmp/autopkgtest.YG0BhS/build.G9O/src/taskflow/engines/action_engine/process_executor.py", line 268, in _calculate_hmac'
    b' mac = hmac.new(auth_key, body).hexdigest()'
    b' File "/usr/lib/python3.8/hmac.py", line 146, in new'
    b' return HMAC(key, msg, digestmod)'
    b' File "/usr/lib/python3.8/hmac.py", line 49, in __init__'
    b" raise ValueError('`digestmod` is required.')"
    b'ValueError: `digestmod` is required.'
    b''

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