Python 3.7: HTTPClientTest.test_get_request_options_with_profile_enabled fails

Hi. Building the python-mistralclient (Rocky, 3.7.0) under Debian Sid, the below unit test fails when using Python 3.7 (but works under Python 2.7). I haven't checked Python 3.6, but it looks like no problem with 3.5 (ie: under Stretch).

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FAIL: mistralclient.tests.unit.test_httpclient.HTTPClientTest.test_get_request_options_with_profile_enabled
mistralclient.tests.unit.test_httpclient.HTTPClientTest.test_get_request_options_with_profile_enabled
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_StringException: Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/mock/mock.py", line 1305, in patched
    return func(*args, **keywargs)
  File "/<<PKGBUILDDIR>>/mistralclient/tests/unit/test_httpclient.py", line 125, in test_get_request_options_with_profile_enabled
    self.assertEqual(signed_data[0], headers['X-Trace-Info'])
  File "/usr/lib/python3/dist-packages/requests/structures.py", line 54, in __getitem__
    return self._store[key.lower()][1]
KeyError: 'x-trace-info'