Asynctest replacement

[andrewwhitehead]

The asynctest library which we use extensively in unit tests is no longer being maintained, and does not function properly under Python 3.8+.

Coincidentally, Python 3.8 with mock 4.0 introduces some tools (IsolatedAsyncioTestCase and AsyncMock) which might be sufficient to replace it. It looks like there is a backwards-compatible stub which could be required for testing in Python 3.6 and 3.7.

There would be some changes required to all async test cases (using asyncSetUp instead of async def setUp for example), and we will probably want to try porting some more advanced test cases first in order to check if any other features of asynctest are required.

[swcurran]

For anyone wanting to contributing and to get a good view on the scope of ACA-Py, this would be a good task.