Comment 6 for bug 1788833

Reviewed: https://review.openstack.org/597421
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=40e18681bc26c1e8f51aa280f600d72ad0846588
Submitter: Zuul
Branch: stable/rocky

commit 40e18681bc26c1e8f51aa280f600d72ad0846588
Author: Stephen Finucane <email address hidden>
Date: Tue Aug 28 17:01:07 2018 +0100

    Don't use '_TransactionContextManager._async'

    In commit 2d532963, all instances of 'async' were replaced with 'async_'
    in preparation for Python 3.7. However, one of these should not have
    been changed as it refers to an oslo.db object attribute. That attribute
    has actually been renamed itself but that rename is only present from
    oslo.db 4.40.0 [1]. Thankfully, an alias to the older name is provided
    so we use that.

    [1] https://github.com/openstack/oslo.db/commit/df6bf34

    Change-Id: I1afd0ba34a9ebcb63edb91e880ef60580befb32e
    Closes-Bug: #1788833
    (cherry picked from commit bd7d991309ea2bea5d175cb1f2710519936fd0c2)