Migrate asynctest to stdlib's AsyncMock

[Gamester17]

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:

• https://github.com/zigpy/bellows/pull/350
• https://github.com/zigpy/zigpy-deconz/pull/125
• https://github.com/zigpy/zigpy/pull/489

[Shulyaka]

I believe this issue can be closed, we are using AsyncMock for python >= 3.8.