Use the original importer for mock. Otherwise, the unit tests that
use importlib with context are not running in an isolated manner,
leading to various transient failures if `stestr` is not run with the
`--isolated` flag.
The unittest.mock._get_target has been changed in Python 3.11, by commit:
ab7fcc8fbd
This change is needed to add support for Python 3.11 and Python 3.12
unit testing.
Change-Id: Ic73f5280116673f771f5460cdfd3702f5fbe16b5
0fded69cb3