Tests: python: use quoted annotations to support python <= 3.6

Python 3.0 introduced syntax for parameter and return type annotations,
as specified in PEP 484. Python 3.6 introduced support for variable type
annotations, as specified in PEP 526.

Upstream recommends using quoted annotations for librairies that need to
support older python versions.

See https://typing.readthedocs.io/en/latest/source/libraries.html#compatibility-with-older-python-versions

Change-Id: Ifc6017a7baa9f0589e85d32005ac9ead18c1c9fb
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>