Migrate to python 3.8 (#1824)

* Migrate to python 3.8

* Fix tests on Py38

* cleanup tests

Co-authored-by: Paulus Schoutsen <balloob@gmail.com>

tests/common.py

@@ -7,20 +7,3 @@ def load_json_fixture(filename):
     """Load a fixture."""
     path = Path(Path(__file__).parent.joinpath("fixtures"), filename)
     return json.loads(path.read_text())
-
-
-def mock_coro(return_value=None, exception=None):
-    """Return a coro that returns a value or raise an exception."""
-    return mock_coro_func(return_value, exception)()
-
-
-def mock_coro_func(return_value=None, exception=None):
-    """Return a method to create a coro function that returns a value."""
-
-    async def coro(*args, **kwargs):
-        """Fake coroutine."""
-        if exception:
-            raise exception
-        return return_value
-
-    return coro