Add new mock features from Python 3.8

[5j9]

In Python 3.8 unittest.mock has had some improvements. It would be nice to see them in this library. The two features that I am currently looking for are:

• MagicMock objects are now awaitable.
• call objects are now named tuples. See the example in documentation:

>>> kall.kwargs is kall[1]
True

[Karthikeyan Singaravelan]

At the core dev sprints I was part of a discussion on this. The upstream now contains several bug fixes along with new features like AsyncMock that makes it Python 3 only. There are syntax level changes like async def that make maintaining this backport to be Python 2 compatible impossible.

At some point maybe as Python 2 reaches end of life after January 1, 2020 there will be a backport of the bug fixes to make a final release. Then there will be a new release with this repo synced with upstream thus essentially dropping Python 2 support and becoming Python 3.5/3.6+ only. Thus users from 3.6 can install this backport and can use features like AsyncMock that are 3.8+ only.

[Chris Withers]

See #476, we can track there.