As indicated in the `asyncmock` project description:

> Mock 4.0+ (included within Python 3.8+) now includes an awaitable mock
> `mock.AsyncMock`. This is recommended for new projects.

Appropriately, update the test code to use `unittest.mock.AsyncMock`
in Python 3.8+, and update the test requirements to pull in `mock >=
4.0` (rather than the obsolete `asyncmock` package) for older Python
versions.