Unit test test_restore_backup_with_body_KeyError fails under Python 3.6

[Thomas Goirand]

Running unit tests under Python 3.6 leads to this error (while it works in Python 2.7):

cinder.tests.unit.api.contrib.test_backups.BackupsAPITestCase.test_restore_backup_with_body_KeyError
----------------------------------------------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    b'Traceback (most recent call last):'
    b' File "/home/zigo/sources/openstack/queens/services/cinder/build-area/cinder-12.0.0~rc1/cinder/tests/unit/api/contrib/test_backups.py", line 1465, in test_restore_backup_with_body_KeyError'
    b" res_dict['badRequest']['message'])"
    b' File "/usr/lib/python3/dist-packages/testtools/testcase.py", line 411, in assertEqual'
    b' self.assertThat(observed, matcher, message)'
    b' File "/usr/lib/python3/dist-packages/testtools/testcase.py", line 498, in assertThat'
    b' raise mismatch_error'
    b'testtools.matchers._impl.MismatchError: !=:'
    b'reference = "Additional properties are not allowed (\'\' was unexpected)"'
    b'actual = "\'restore\' is a required property"'
    b''
    b''

Captured pythonlogging:
~~~~~~~~~~~~~~~~~~~~~~~
    b'2018-02-16 10:44:32,625 INFO [cinder.rpc] Automatically selected cinder-scheduler objects version 1.35 as minimum service version.'
    b'2018-02-16 10:44:32,627 INFO [cinder.rpc] Automatically selected cinder-scheduler RPC version 3.10 as minimum service version.'
    b'2018-02-16 10:44:32,630 INFO [cinder.rpc] Automatically selected cinder-volume objects version 1.35 as minimum service version.'
    b'2018-02-16 10:44:32,632 INFO [cinder.rpc] Automatically selected cinder-volume RPC version 3.15 as minimum service version.'
    b'2018-02-16 10:44:32,634 INFO [cinder.rpc] Automatically selected cinder-backup objects version 1.35 as minimum service version.'
    b'2018-02-16 10:44:32,637 INFO [cinder.rpc] Automatically selected cinder-backup RPC version 2.1 as minimum service version.'
    b'2018-02-16 10:44:32,790 INFO [cinder.api.openstack.wsgi] POST http://localhost/v2/89afd400-b646-4bbc-b12b-c0a4d63e5bd3/backups/7f1821ac-cb5d-46d2-beda-8bdc899f9546/restore'
    b'2018-02-16 10:44:32,791 INFO [cinder.api.openstack.wsgi] http://localhost/v2/89afd400-b646-4bbc-b12b-c0a4d63e5bd3/backups/7f1821ac-cb5d-46d2-beda-8bdc899f9546/restore returned with HTTP 400'
    b''