Use stdlib AsyncMock when available

[mgorny]

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.

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[richardm-stripe]

Hello @mgorny. Thank you for the contribution.

I agree with this change. I think when the time comes and we've dropped support for python < 3.8 having the code like this will make it easier to make the appropriate update.

Can you sign the CLA in the github comment from @CLAassistant? (looks good)

[mgorny]

Thanks!