Migrate asynctest to stdlib's AsyncMock

[Andreas Setterlind]

Same as zigpy/bellows#339 and zigpy/zigpy#475 as reported by @fabaff of Home Assistant / hass.io :

Asynctest doesn't work with Python 3.8 and above. It's the same issue as for zigpy (zigpy/zigpy#475).

Update: FYI, bellows, zigpy-deconz, and zigpy now changed to AsyncMock:

• zigpy/bellows#350
• zigpy/zigpy-deconz#125
• zigpy/zigpy#489