Migrate asynctest to stdlib's AsyncMock

[fabaff]

Asynctest doesn't work with Python 3.8 and above.

Asynctest upstream is aware of this breakage and recommends using the built-in AsyncMock instead.

See also Martiusweb/asynctest#144 and Martiusweb/asynctest#126

[puddly]

What's the recommended approach for mocking coroutines on Python 3.7? Is there any backport of AsyncMock other than the one in asynctest?

try:
    # Python 3.8 already has this
    from mock import AsyncMock as CoroutineMock
except ImportError:
    from asynctest import CoroutineMock

[Adminiuga]

we could take HA's core approach and just import CoroutineMock and/or mock from the local asyncmock module
https://github.com/home-assistant/core/blob/dev/tests/async_mock.py