asynctest library is not compatible with Python 3.8 and above

[anjandev]

asynctest library is not compatible with Python 3.8 and above: https://github.com/Martiusweb/asynctest/issues/144.

Upstream has abandoned it and recommends using the built-in AsyncMock instead: https://github.com/Martiusweb/asynctest/issues/158#issuecomment-785872568.

We should redo the testsuite to use AsyncMock.

[rndusr]

Thanks for the heads up.

While we're at it, we should also split some submodules into separate projects and use a different TUI library, probably Textual. https://github.com/willmcgugan/textual