py37: deal with Exception repr changes
py37: deal with Exception repr changes

Under Python 3.7, a trailing comma is no longer added to
the init parameters generated by a repr() call:

    >>> repr(Exception('It Works'))
    "Exception('It Works')"

vs

    >>> repr(Exception('It Works'))
    "Exception('It Works',)"

Support pre and post Python 3.7 formats in test cases.

Change-Id: Idff8c841fd6c489e887bd72b49e91ef6f0e2d4f3
Closes-Bug: #1784487

openstack/oslo.vmware