Make AsyncMock stubs available for python 3.6+

[GDYendell]

• AsyncMock is available in 3.6+ with the third party mock package
• The 3.8+ restriction only exists in stdlib unittest.mock

[GDYendell]

Fixes #5947

[GDYendell]

I initially just removed the version check, but the python2 section of the tests failed, so I have now changed the check to >3.6, as the mock package states. However, it also doesn't actually claim compatibility with python 2, so I wonder if the correct fix is to again remove the version check entirely and instead remove the python2 = true key.

[srittau]

The homepage explicitly says "This package [...] compatible with Python 3.6 and up." So I think we should remove the python2 key as you suggest.

[srittau]

Thanks!